Nothing can be known to exist except by the help of experience. That is to say, if we wish to prove that something of which we have no direct
experience exists, we must have among our premisses the existence of one or more things of which we have direct experience. Our belief that the
Emperor of China exists, for example, rests upon testimony, and testimony consists, in the last analysis, of sense-data seen or heard in reading or
being spoken to. Rationalists believed that, from general consideration as to what must be, they could deduce the existence of this or that in the
actual world. In this belief they seem to have been mistaken. All the knowledge that we can acquire a priori concerning existence seems to be
hypothetical: it tells us that if one thing exists, another must exist, or, more generally, that if one proposition is true another must be true. This
is exemplified by principles we have already dealt with, such as 'if this is true, and this implies that, then that is true', of 'if this and that
have been repeatedly found connected, they will probably be connected in the next instance in which one of them is found'. Thus the scope and power of
a priori principles is strictly limited. All knowledge that something exists must be in part dependent on experience. When anything is known
immediately, its existence is known by experience alone; when anything is proved to exist, without being known immediately, both experience and a
priori principles must be required in the proof. Knowledge is called empirical when it rests wholly or partly upon experience. Thus all knowledge
which asserts existence is empirical, and the only a priori knowledge concerning existence is hypothetical, giving connexions among things that exist
or may exist, but not giving actual existence.
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