When I was writing my book on existence I was troubled by the question as to how one knows that there are contingent beings. For I took it as given that there are, just as I took it as given that things exist. But one philosopher's datum is another's theory, and I was hoping to begin my metaphysical ascent from indubitable starting points. So it bugged me: how do I know that this coffee cup is a contingent being? Given that it exists, how do I know that it exists contingently? I satisfied my scruples by telling myself that I was writing about the metaphysics of existence and that concerns with its epistemology could be reserved for a later effort. What exactly is the problem? Let's begin with a couple of definitions:
D1. X is contingent =df possibly (x exists) & possibly (x does not exist).
The possibility at issue is non-epistemic and broadly logical. And note that the definiens of (D1) is not to be confused with 'possibly (x exists & x does not exist)' which is necessarily false.
D2. X contingently exists =df x exists & possibly (x does not exist).
Note that to say that x exists contingently is not to say that x depends for its existence on something else; it is merely to say that x exists and that there is no broadly logical (metaphysical) necessity that x exist. Suppose exactly one thing exists, an iron sphere. Intuitively, the sphere is contingent despite there being nothing on which it depends for its existence. For though it exists, it might not have.
Note also that to say that x exists contingently is not to say that x is actual at some times and not actual at other times. (Even if everything that contingently exists exists at some times but not at all times, the contingency of what contingently exists does not consist in its existing at some but not all times.) If one said that contingency is existence at some but not all times, then one would have to say that x exists necessarily just in case x exists at all times. Something that exists at all times, however, could well be contingent in a clear sense of this term, namely, possibly nonexistent. For example, suppose the physical universe always existed and always will exist. It doesn't follow that it necessarily exists (is impossibly nonexistent). It would remain a contingent fact that it exists at all in the D2 sense. And then there that are items that are not in time at all: numbers, Fregean propositions, and other 'abstracta.' They exist necessarily without being temporally qualified. Their necessity is not their existence at all times.
For example, my coffee cup exists now -- how I know this is a separate epistemological question that I here ignore -- but is possibly such that it does not exist now, where 'now' picks out the same time. But how do I know that the cup is now possibly nonexistent? That's my problem.
This is a variant of the problem of modal knowledge. (See Notes on Van Inwagen on Modal Epistemology.) The cup is full, but it might not have been. It is full of coffee, but it might have been full of whisky. It is two inches from the ashtray, but it might have been three inches from it. It exists now but it might not have existed now. It has existed for 20 years; it might never have existed at all. And so on. I can see that the cup is full, and I can taste that it contains coffee and not whisky. But I cannot see or taste what doesn't exist (assuming that 'see' is being used as a verb of success), and the cup's being empty or the cup's containing whisky are non-obtaining states of affairs. Thus there seems to be nothing for my modal knowledge to 'grab onto.'
If I know that the cup exists contingently, then I know that it is possibly nonexistent. But how do I know the latter?
"You know it from your ability to conceive, without contradiction, of the cup's nonexistence." This is not a good answer. First of all, conceivability (without contradiction) does not entail possibility. Example here. Does the conceivability of p raise the probability of p's being possible? This is a strange notion. Discussion here.
If conceivability neither entails nor probabilifies possibility, then my question returns in full force: how does one know, of any being, that it is a contingent being?
"Well, you know from experience that things like coffee cups come into existence and pass out of existence. If you know that, then you know that such things do not exist of metaphysical necessity. For what exists of metaphysical necessity exists at all times, if it exists in time at all, and your coffee cup, which exists in time, does not exist at all times. Now what does not exist of metaphysical necessity is metaphysically contingent. Therefore, you know that coffee cups and such are contingent existents."
This argument may do the trick. To test it, I will set it forth as rigorously as possible. To save keystrokes I omit universal quantifiers.
1. If x is a material thing, and x does not exist at all times, then x is not a necessary being (one whose nonexistence is broadly-logically impossible).
2. If x is not a necessary being, then x is either a contingent being or an impossible being.
Therefore
3. If x is a material thing, and x does not exist at all times, then x is either a contingent being or an impossible being.
4. My coffee cup is a material thing and it does not exist at all times.
Therefore
5. My cup is either a contingent being or an impossible being.
6. If x exists, then x is not impossible.
7. My cup exists.
Therefore
8. My cup is a contingent being.
9. I know that (8) because I know each of the premises, and (8) follows from the premises.
The inferences are all valid, and the only premise that might be questioned is (1). To refute (1) one needs an example of a material being that does not exist at all times that is a necessary being. But I can't think of an example.
The argument just given seems to be a rigorous proof that there is at least one contingent (possibly nonexistent) existent. But does it show that this existent is possibly nonexistent at each time at which it exists? (The latter is the question I posed above.)
Would it make sense to say that my cup, though not a necessary being, is necessarily existent at each time at which it exists? If that makes sense, then my cup is contingent in that it might not have existed at all, but not contingent in the sense that at each time at which it exists it is possibly nonexistent. Are these two propositions consistent:
a. x is contingent in that it might not have existed at all
and
b. x is not contingent in the sense of being possibly nonexistent at each time at which it exists?
If (a) and (b) are consistent, then it appears that I have not proven that my cup is contingent in the sense of being possibly nonexistent at each time at which it exists. For then the above argument shows merely that the cup is contingent in that it might not have existed at all.
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