存在論とクワイン

Abstract

We, as well as Quine, know for sure that there are physical objects like dogs, horses, milk, stone, etc. in this world. Nobody would disagree with this. But, if we move on to the existence of abstract objects like attributes and relations, then it is not so easy to straightforwardly say that they are, or that they exist, in the world. For ex-ample, the attribute 'being human' does not exist in the same way as a man like Quine exists in the world. We say, however, that John is human, which means that John has the attribute 'being human.' It seems to me that in order for us to say so, there must be such an attribute as 'being human' in some way or other. The attribute 'being human' is some thing which all people have and we can recognize that they have it. Our recognition of that attribute suggests its existence in some sense. There is another type of thing such as Pegasus, unicorns, Sherlock Holmes, etc. This type of entity appears in fictions and stories. Though they are fictitious entities, we cannot claim that they do not exist in any sense. The statement 'Pegasus exists' is not completely false, though we do not think Pegasus exists in the real world like actual physical objects do. 'Pegasus exists' could mean that Pegasus exists in Greek mythology. According to Quine, mathematical entities like classes, numbers, and functions are considered to be of the same type of entity as that of attributes and propositions. Namely, he does not think that classes, numbers, and functions exist in the same sense that physical objects do. I agree with him on this point since I clearly see the difference between the existence of classes, numbers, etc. and the existence of actual physic-al objects. For example, the class {New York, Boston, Chicago} of cities cannot be found in the real world. What we can find is the three cities which exist in the United States. However, we feel no oddity to the existence claims of classes and numbers mainly because mathematics greatly prevails in our ordinary life; it is quite normal to claim that there is a number between 8 and 10. Though classes, numbers, and functions can be said to exist, the kind of being of those abstract entities is very different from the kind of being of actual physical objects.