That is to say, there are such things as hallucinations.
The criterion of vividness will not, therefore, always serve to distinguish between what is given in the sense and what is only imagined.

And, indeed, it becomes evident, upon reflection, that we do not actually make it our ultimate test.

We may be quite willing to admit that faint sensations may come to be confused with what is imagined, with "ideas," but we always regard such a confusion as somebody's error.

We are not ready to admit that things perceived faintly are things imagined, or that vivid "ideas" are things perceived by sense.
Let us come back to the illustrations with which we started.

How do I know that I perceive the desk before me; and how do I know that, sitting here, I imagine, and do not see, the front door of the house?
My criterion is this: when I have the experience I call "seeing my desk," the bit of experience which presents itself as my desk is in a certain setting.