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Most positions that can be taken with regards to any of the following questions are endorsed by one or another notable philosopher.
It is often difficult to frame the questions in a non-controversial manner.
Being, existence and reality
The nature of Being is a perennial topic in metaphysics.
The 20th century philosopher Heidegger thought previous philosophers have lost sight of the question of Being (qua Being) in favour of the questions of beings (existing things), so that a return to the Parmenidean approach was needed.
An ontological catalogue is an attempt to list the fundamental constituents of reality.
Existence, that something is, has been contrasted with essence, the question of what something is.
Reflections on the nature of the connection and distinction between existence and essence dates back to Aristotle‘s Metaphysics, and it found one of its later most influential interpretations in the ontology of the eleventh century metaphysician Avicenna (Ibn Sina).
Since existence without essence seems blank, it is associated with nothingness by philosophers such as Hegel.
Empirical and conceptual objects
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; the former objects are called particulars.
Particulars are said to have attributes, e.g. size, shape, color, location and two particulars may have some such attributes in common.
Such attributes, are also termed Universals or Properties; the nature of these, and whether they have any real existence and if so of what kind, is a long-standing issue, realism and nominalism representing opposing views.
David Armstrong holds that universals exist in time and space but only at their instantiation and their discovery is a function of science.
Others maintain that particulars are a bundle or collection of properties (specifically, a bundle of properties they have).
Some authors claim (or at least presuppose) that taxa are real entities, that to say that an animal is included in Mammalia (the scientific name for the mammal group) is to say that it bears a certain relation to Mammalia, an abstract object.
Advocates of phylogenetic nomenclature, a more nominalistic view, oppose this reading; in their opinion, calling an animal a mammal is a shorthand way of saying that it is descended from the last common ancestor of, say, humans and platypuses.
Cosmology and cosmogony
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 times it addresses questions about the Universe which are beyond the scope of the physical sciences.
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? (see monism, pantheism, emanationism and creationism)
- What are the ultimate material components of the Universe? (see mechanism, dynamism, hylomorphism, atomism)
- What is the ultimate reason for the existence of the Universe? Does the cosmos have a purpose? (see teleology)
Determinism and free will
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.
If they believe in determinism, they will therefore believe free will to be an illusion, a position known as Hard Determinism.
Others, labeled Compatibilists (or “Soft Determinists”), believe that the two ideas can be coherently reconciled.
Identity and change
The Greeks took some extreme positions on the nature of change: Parmenides denied that change occurs at all, while Heraclitus thought change was ubiquitous: “[Y]ou cannot step into the same river twice.”
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.
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.
It is worth noting, however, that the grounds necessary to ensure validity to the proposed theory’s veridical nature were not scientific, but just as philosophical as those traditions espoused by Thales and Anaximander.
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 mind as a whole.
René Descartes proposed substance dualism, a theory in which mind and body are essentially 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.
Idealism is also a common theme in Eastern philosophy.
Alfred North Whitehead was a twentieth-century exponent of this approach.
It claims that existence consists of a single substance that 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. Type identity theory, token identity theory, functionalism, reductive physicalism, nonreductive physicalism, eliminative materialism, anomalous 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, Fred Dretske, Douglas Hofstadter, Jerry Fodor, David Lewis, Thomas Nagel, Hilary Putnam, John Searle, John Smart, Ludwig Wittgenstein, and Fred Alan Wolf.
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.
A possible fact 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 law of identity, a putative “first principle”.
Aristotle describes the principle of non-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.”
Religion and spirituality
Theology is the study of a god or gods and the nature of the divine.
Whether there is a god (monotheism), many gods (polytheism) or no gods (atheism), or whether it is unknown or unknowable whether any gods exist (agnosticism; apophatic theology), and whether the Divine intervenes directly in the world (theism), or its sole function is to be the first cause of the universe (deism); these and whether a God or gods and the World are different (as in panentheism and dualism), or are identical (as in pantheism), are some of the primary metaphysical questions concerning philosophy of religion.
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.
Space and time
A traditional realist position in ontology is that time and space have existence apart from the human mind.
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.
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 direction of time, also known as “time’s arrow”, is also a puzzle, although physics is now driving the debate rather than philosophy.
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.
Broadly speaking, endurantists hold that a whole object exists at each moment of its history, and the same object exists at each moment.