Thus, the ghost which is seen by the terrified peasant at midnight, but which cannot be photographed, we may unhesitatingly set down as something imagined and not really seen.
All our sensations are, therefore, experiences which take their place in a certain setting.

This is our ultimate criterion.

We need not take the word of the philosopher for it.

We need only reflect, and ask ourselves how we know that, in a given case, we are seeing or hearing or touching something, and are not merely imagining it.

In every case, we shall find that we come back to the same test.