philosopher. It is often difficult to frame the questions in a non-controversial manner.
Mind and matter
The nature of matter was a problem in its own right in early philosophy. Aristotle himself introduced the idea of matter in general to the Western world, adapting the term hyle which originally meant "lumber". Early debates centered on identifying a single underlying principle. Water was claimed by Thales, Air by Anaximenes, Apeiron (the Boundless) by Anaximander, Fire by Heraclitus. Democritus conceived an atomic theory many centuries before it was accepted by modern science.
Philosophers now look to empirical science for insights into the nature of matter.
The nature of the mind and its relation to the body has been seen as more of a problem as science has progressed in its mechanistic understanding of the brain and body. Proposed solutions often have ramifications about the nature of reality as a whole. René Descartes proposed substance dualism, a theory in which mind and body are essentially quite different, with the mind having some of the attributes traditionally assigned to the soul, in the seventeenth century. This creates a conceptual puzzle about how the two interact (which has received some strange answers, such as occasionalism). Evidence of a close relationship between brain and mind, such as the Phineas Gage case, have made this form of dualism increasingly unpopular.
Another proposal discussing the mind-body problem is idealism, in which the material is sweepingly eliminated in favor of the mental. Idealists, such as George Berkeley, claim that material objects do not exist unless perceived and only as perceptions. The "German idealists" such as Fichte, Hegel and Schopenhauer took Kant as their starting-point, although it is debatable how much of an idealist Kant himself was. Idealism is also a common theme in Eastern philosophy. Related ideas are panpsychism and panexperientialism which say everything has a mind rather than everything exists in a mind. Alfred North Whitehead was a twentieth-century exponent of this approach.
Idealism is a monistic theory, in which there is a single universal substance or principles. Neutral monism, associated in different forms with Baruch Spinoza and Bertrand Russell is a theory which seeks to be less extreme than idealism, and to avoid the problems of substance dualism. It claims that existence consists of a single substance, which in itself is neither mental nor physical, but is capable of mental and physical aspects or attributes – thus it implies a dual-aspect theory.
For the last one hundred years, the dominant metaphysics has without a doubt been materialistic monism. Science has demonstrated many ways in which mind and brain interact, but the exact nature of the relationship is still open to debate. Type identity theory, token identity theory, functionalism, reductive physicalism, nonreductive physicalism, eliminative materialism, anomolous monism, property dualism, epiphenomenalism and emergence are just some of the candidates for a scientifically-informed account of the mind. (It should be noted that while many of these positions are dualisms, none of them are substance dualism.)
Prominent recent philosophers of mind include David Armstrong, Ned Block, David Chalmers, Patricia and Paul Churchland, Donald Davidson, Daniel Dennett, Jerry Fodor, David Lewis, Thomas Nagel, Hilary Putnam, John Searle, John Smart and Ludwig Wittgenstein.
Objects and their properties
The world seems to contain many individual things, both physical, like apples, and abstract such as love and the number 3. Such objects are called particulars. Now, consider two apples. There seem to be many ways in which those two apples are similar, they may be approximately the same size, or shape, or color. They are both fruit, etc. One might also say that the two apples seem to have some thing or things in common. Universals or Properties are said to be those things.
Metaphysicians working on questions about universals or particulars are interested in the nature of objects and their properties, and the relationship between the two. For instance, one might hold that properties are abstract objects, existing outside of space and time, to which particular objects bear special relations. Others maintain that what particulars are is a bundle or collection of properties (specifically, a bundle of properties they have).
Identity and change
The Greeks took some extreme positions on the nature of change: Parmenides denied that change occurs at all, while Heracleitus thought change was ubiquitous: "[Y]ou cannot step into the same river twice".
Identity, sometimes called Numerical Identity, is the relation that a "thing" bears to itself, and which no "thing" bears to anything other than itself (cf. sameness). According to Leibniz, if some object x is identical to some object y, then any property that x has, y will have as well. However, it seems, too, that objects can change over time. If one were to look at a tree one day, and the tree later lost a leaf, it would seem that one could still be looking at that same tree. Two rival theories to account for the relationship between change and identity are Perdurantism, which treats the tree as a series of tree-stages, and Endurantism which maintains that the tree -- the same tree -- is present at every stage in its history.
Space and time
In the Middle Ages, Saint Augustine of Hyppo asked the fundamental question about the nature of time. A traditional realist position in ontology is that time and space have existence apart from the human mind. Idealists, including Kant claim that space and time are mental constructs used to organise perceptions, or are otherwise unreal.
Suppose that one is sitting at a table, with an apple in front of him or her; the apple exists in space and in time, but what does this statement indicate? Could it be said, for example, that space is like an invisible three-dimensional grid in which the apple is positioned? Suppose the apple, and all physical objects in the universe, were removed from existence entirely. Would space as an "invisible grid" still exist? René Descartes and Leibniz believed it would not, arguing that without physical objects, "space" would be meaningless because space is the framework upon which we understand how physical objects are related to each other. Newton, on the other hand, argued for an absolute "container" space. The pendulum swung back to relational space with Einstein and Ernst Mach.
While the absolute/relative debate, and the realism debate are equally applicable to time and space, time presents some special problems of its own. The flow of time has been denied in ancient times by Parmenides and more recently by J. M. E. McTaggart in his paper The Unreality of Time.
The direction of time, also known as "time's arrow", is also a puzzle, although physics is now driving the debate rather than philosophy. It appears that fundamental laws are time-reversible and the arrow of time must be an "emergent" phenomenon, perhaps explained by a statistical understanding of thermodynamic entropy.
Common-sense tells us that objects persist across time, that there is some sense in which you are the same person you were yesterday, in which the oak is the same as the acorn, in which you perhaps even can step into the same river twice. Philosophers have developed two rival theories for how this happens, called "endurantism" and "perdurantism". Broadly speaking, endurantists hold that a whole object exists at each moment of its history, and the same object exists at each moment. Perdurantists believe that objects are four-dimensional entities made up of a series of temporal parts like the frames of a movie.
Religion and spirituality
Theology is the study of God and the Nature of the Divine. Is there a God (monotheism), many Gods (polytheism) or no Gods (atheism)? Does the Divine intervene directly in the world (theism), or is its sole function to be the first cause of the universe (deism)? Are God and the World different (panentheism, dualism) or are they identical (pantheism)? These are the primary metaphysical questions concerning theologians.
Within the standard Western philosophical tradition, theology reached its peak under the medieval school of thought known as scholasticism, which focused primarily on the metaphysical aspects of Christianity. While the work of the scholastics has been largely eclipsed in the wake of modern philosophy, key figures such as Thomas Aquinas still play an important role in the philosophy of religion.
Necessity and possibility
Metaphysicians investigate questions about the ways the world could have been. David Lewis, in "On the Plurality of Worlds," endorsed a view called Concrete Modal realism, according to which facts about how things could have been are made true by other concrete worlds, just like ours, in which things are different. Other philosophers, such as Gottfried Leibniz, have dealt with the idea of possible worlds as well. The idea of necessity is that any necessary fact is true across all possible worlds; that is, we could not imagine it to be otherwise. A possible fact is one that is true in some possible world, even if not in the actual world. For example, it is possible that cats could have had two tails, or that any particular apple could have not existed. By contrast, certain propositions seem necessarily true, such as analytic propositions, e.g. "All bachelors are unmarried." The particular example of analytic truth being necessary is not universally held among philosophers. A less controversial view might be that self-identity is necessary, as it seems fundamentally incoherent to claim that for any x, it is not identical to itself; this is known as the principle of contradiction. Aristotle describes the principle of contradiction, "It is impossible that the same quality should both belong and not belong to the same thing . . . This is the most certain of all principles . . . Wherefore they who demonstrate refer to this as an ultimate opinion. For it is by nature the source of all the other axioms."
Abstract objects and mathematics
Some philosophers endorse views according to which there are abstract objects such as numbers, or Universals. (Universals are properties that can be instantiated by multiple objects, such as redness or squareness.) Abstract objects are generally regarded as being outside of space and time, and/or as being causally inert. Mathematical objects and fictional entities and worlds are often given as examples of abstract objects. The view that there really are no abstract objects is called nominalism. Realism about such objects is exemplified by Platonism. Other positions include moderate realism, as espoused by Aristotle, and conceptualism.
The philosophy of mathematics overlaps with metaphysics because some positions are realistic in the sense that they hold that mathematical objects really exist, whether transcendentally, physically, or mentally. Platonic realism holds that mathematical entities are a transcendent realm of non-physical objects. The simplest form of mathematical empiricism claims that mathematical objects are just ordinary physical objects, i.e. that squares and the like physically exist. Plato rejected this view, among other reasons, because geometrical figures in mathematics have a perfection that no physical instantiation can capture.
Modern mathematicians have developed many strange and complex mathematical structures with no counterparts in observable reality, further undermining this view. The third main form of realism holds that mathematical entities exist in the mind. However, given a materialistic conception of the mind, it does not have the capacity to literally contain the many infinities of objects in mathematics. Intuitionism, inspired by Kant, sticks with the idea that "there are no non-experienced mathematical truths". This involves rejecting as intuitionistically unacceptable anything that cannot be held in the mind or explicitly constructed. Intuitionists reject the law of the excluded middle and are suspicious of infinity, particularly of transfinite numbers.
Other positions such as formalism and fictionalism that do not attribute any existence to mathematical entities are anti-realist.
Determinism and free will
Determinism is the philosophical proposition that every event, including human cognition, decision and action, is causally determined by an unbroken chain of prior occurrences. It holds that no random, spontaneous, mysterious, or miraculous events occur. The principal consequence of the deterministic claim is that it poses a challenge to the existence of free will.
The problem of free will is the problem of whether rational agents exercise control over their own actions and decisions. Addressing this problem requires understanding the relation between freedom and causation, and determining whether the laws of nature are causally deterministic. Some philosophers, known as Incompatibilists, view determinism and free will as mutually exclusive. If they believe in determinism, they will therefore believe free will to be an illusion, a position known as Hard Determinism. Proponents range from Baruch Spinoza to Ted Honderich.
Others, labeled Compatibilists (or "Soft Determinists"), believe that the two ideas can be coherently reconciled. Adherents of this view include Thomas Hobbes and many modern philosophers.
Incompatibilists who accept free will but reject determinism are called Libertarians, a term not to be confused with the political sense. Robert Kane is one of the few modern defenders of this theory.
It is a popular misconception that determinism necessarily entails that humanity or individual humans have no influence on the future and its events, a position known as Fatalism). Determinists, however, believe that the level to which human beings have influence over their future is itself dependent on present and past.
Cosmology and cosmogony
Cosmology is the branch of metaphysics that deals with the world as the totality of all phenomena in space and time. Historically, it has had quite a broad scope, and in many cases was founded in religion. The ancient Greeks did not draw a distinction between this use and their model for the cosmos. However, in modern use it addresses questions about the Universe which are beyond the scope of science. It is distinguished from religious cosmology in that it approaches these questions using philosophical methods (e.g. dialectics). Cosmogony deals specifically with the origin of the universe.
Modern metaphysical cosmology and cosmogony try to address questions such as:
What is the origin of the Universe? What is its first cause? Is its existence necessary?
What are the ultimate material components of the Universe?
What is the ultimate reason for the existence of the Universe? Does the cosmos have a purpose?
Metaphysics has been attacked, at different times in history, as being futile and overly vague. David Hume went so far as to write: If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.
– An Enquiry Concerning Human Understanding
Immanuel Kant prescribed a limited role to the subject and argued against knowledge progressing beyond the world of our representations, except to knowledge that the noumena exist: though we cannot know these objects as things in themselves, we must yet be in a position at least to think them as things in themselves; otherwise we should be landed in the absurd conclusion that there can be appearance without anything that appears.
– Critique of Pure Reason pp. Bxxvi-xxvii
A.J. Ayer is famous for leading a "revolt against metaphysics," where he claimed that its propositions were meaningless in his book "Language, Truth and Logic". Ayer was a defender of verifiability theory of meaning. British universities became less concerned with the area for much of the mid 20th century. However, metaphysics has seen a reemergence in recent times among philosophy departments due to the failure of verificationism.
A more nuanced view is that metaphysical statements are not meaningless statements, but rather that they are generally not fallible, testable or provable statements. That is to say, there is no valid set of empirical observations nor a valid set of logical arguments, which could definitively prove metaphysical statements to be true or false. Hence, a metaphysical statement usually implies an idea about the world or about the universe, which may seem reasonable but is ultimately not empirically verifiable. That idea could be changed in a non-arbitrary way, based on experience or argument, yet there exists no evidence or argument so compelling that it could rationally force a change in that idea, in the sense of definitely proving it false.
Aristotle, David Malet Armstrong, St. Thomas Aquinas, St. Augustine, Avicenna (Ibn Sina), George Berkeley, Boethiu, David Chalmers, Roderick Chisholm, Donald Davidson, Gilles Deleuze, Jacques Derrida, René Descartes, René Guénon, Georg W. F. Hegel, Martin Heidegger, David Hume, Edmund Husserl, Peter van Inwagen, Immanuel Kant, Mahmoud Khatami, Jaegwon Kim, Saul Kripke, Emmanuel Lévinas, Gottfried Leibniz, David Lewis, J. M. E. McTaggart, George Edward Moore, Thomas Nagel, Friedrich Nietzsche, Derek Parfit, Alvin Plantinga, Plato, Plotinus, Hilary Putnam, Willard V. O. Quine, Nicholas Rescher, Richard Rorty, Bertrand Russell, Jean-Paul Sartre, Arthur Schopenhauer, John Duns Scotus, Wilfrid Sellars, J. J. C. Smart, Baruch Spinoza, Rudolf Steiner, Richard Swinburne, Alfred N. Whitehead, Ludwig Wittgenstein