The word "being" is an understanding that which in the twentieth century is identified with reality. What does the word "real" mean? The sphere of the real is defined as the sphere of existence that is totally independent of the human mind. The familiar distinction between appearance and reality calls our attention to the fact that there is sometimes a difference between the way things appear to us and way they really are -- the character they have apart from our cognition of them.
Immanuel Kant was correct in thinking that the history of philosophy would be divided into two periods -- before and after Kant. In the two thousand or more years before Kant's Copernican revolution in philosophy, no one questioned the existence of an independent reality and, except for a few extreme skeptics, no one doubted that reality was both knowable and intelligible. Beginning with Kant, such realism ceases to be relevant, and for the first time what might be called "ontological idealism": takes its place. Before Kant, no other philosopher was an idealist Idealism is a peculiarly modern error, one that is widely prevalent today and one for which we are indebted to Kant.
This is not the place to explain how Kant made this mistake, except to say that it would not have occurred to him did he not think that Newton's physics and Euclid's geometry expounded the truth with certitude, and so they needed to be defended against Hume's mistreatment of them. Suffice it to say, now that Einstein has supplanted Newton and now that we have non-Euclidean geometry, Kant's attempt to give Euclid and Newton certitude is an ingeniously contrived theory of how the innate structure of the human mind imposes on our experience the form of space that is Euclidean and the kind of causality that explains Newton's celestial mechanics. But it is nothing more than an ingenious contrivance and it can be dismissed as no longer necessary or sustainable.
Kant does not deny the existence of reality. The thing in itself -- the Dinge an sich -- exists, but it is unknowable. Being independent of the human mind, it lies outside all possible experience, which is given its shape and all its features by the human mind.
In the period before Kant, the treatment of being took its main terms from Aristotle's Metaphysics and from the De Ente et Essentia ("Of Being and Essence") of Thomas Aquinas. Today, when the leading philosophers are idealists rather than realists, we are mainly concerned with the way things appear to be as opposed to the way they really are.
We are, therefore, called upon to face a problem that Sir Arthur Eddington states for us in the opening chapter of his Gifford Lectures on The Nature of the Physical World. The table in front of him, he says, is solid and impenetrable to his hands that lean upon it. But he tells us that, from the physicist's point of view, it is nothing but a field of empty space, a void in which elementary particles are moving about with great speed.
How can the same table be both what it appears to be to our ordinary sense perception and what it really is according to the physicist's theory of it? I will summarize my solution of the problem. The elementary particles exist really only when they exist in a cyclotron, not when they are organized as the constituents of all the physical things that are the objects of sense-perception. In the latter case, the elementary particles are only virtually present, and that virtual existence can be turned into actual existence only by destroying the physical thing in which they are virtually present.
Ever since Kant, philosophers have vainly tried to prove the real existence of the external world -- the world outside our minds. A correct understanding of perception shows why such efforts are vain as well as unnecessary. Except for the mental act of perception, all other acts of the mind, such as imagination, memory, and conception, present us with objects concerning which we must ask whether, in addition to being objects of the mind, they also exist in reality. But in the one case of perception, we cannot separate our having the perceptual object before our minds from asserting that it also really exists.
If that were not so, there would be no distinction between hallucinating and perceiving. Hallucination is pathological. Normal perception is always the perception of something that has existence in reality. If we do not assert that the perceptual object also really exists as a perceptible thing, we cannot say that we are perceiving it. No proof of an external world is necessary.
Existence has three modalities. The first is real existence, existence independent of the human mind and unaffected by it. The second modality is subjective existence. The contents of the human mind, its sensations, perceptions, images, memories, and concepts, have existence in your mind and mine. Since you and I really exist, subjective existence is also one form of real existence.
There is a third modality -- a third mode of existence that is neither totally independent of the human mind nor totally dependent on the individual mind. This third mode is the existence of all the objects of the human mind except perceptual objects. Other than perceptual objects, we must always ask whether they have real existence as well as objective existence, that is, existence as intended objects, the objects that the human mind intends or means.
This third mode of existence, is a middle ground between real existence and subjective existence. Real existence is existence independent of mind: it is the existence that physical things had before there were human beings on earth. Intentional existence is not independent of the human mind, but it is also not dependent on the existence of any one individual mind, as subjective existence is.
Objects that exist for two or more minds, objects that they can discuss with each other, have intentional existence. If there were no minds on earth, there would be no objects that had intentional existence. To summarize this middle ground between real existence and mental existence. It consists in (1) not being dependent on the acts of any particular human mind, and in this respect it differs from subjective existence. And (2) not being independent of the human mind in general, and in this respect it differs from real existence. It is a mode of existence that depends on there being some individual minds at work.
Another distinction with which we must deal is that between being and becoming, between the mutable being of all things subject to change, and the immutable being of that which is timeless and unchangeable. This is eternal which is beyond time and change. In the realm of change and time, past events exist only as objects remembered, and future events exist only as objects imagined.
Here we must take account of necessary and contingent beings; and among contingent being, some are subject to transformation, as a human being is when an individual dies and turns into dust and ashes. A contingent being may be radically contingent; only the cosmos as a whole is radically contingent. The cosmos can be otherwise in character; and it is self-evidently true that which is capable of being otherwise is also capable of not being at all -- of being replaced by the null and void, by nothingness.
Finally we must consider distinctions between the possible and the impossible. The latter is that which is incapable of being. In the sphere of the possible, we must further distinguish between entia reale and entia rationis. The latter are fictions of the mind, such things as mermaids centaurs, and unicorns, and also such fictional characters as Antigone and Hamlet.
We should be shocked by Stephen Hawking's bold as well as erroneous statement that what is not measurable by the physicist has no existence in reality. In the same book in which Professor Hawking makes that statement, he also frequently refers to the mind of God, which is certainly not measurable by the physicist. Either he is referring to something that has no reality, or he is wrong in saying that those aspects of time which are not measurable by the physicist have no reality. (See Time)
Adler's Philosophical Dictionary (1995)
Great Ideas from the Great Books (1963)
by Mortimer J. Adler