In the preceding chapter we have seen how Kant holds that it is verified that whatever can be known to exist must be a thing or a property of a thing. When a man steps from his house and finds his sidewalk wet, he knows a certain property of it. When he knows that it is wet because it has been raining, he does not know some further property of it. In so far as anyone knows what the cause of something is, what he then knows about it is not some property of it. In the present chapter a further metaphysical principle will be brought under scrutiny. Must there be a reason for whatever exists? Before considering what means Kant holds are available for making out whether this principle is true, we shall first attend to the light which he has to throw on two preliminary questions: How do we ascertain what the cause of something is? Just what are we thinking about something when we think what its cause is?