Neither of these suppositions is strictly true.

Although in general we may abstract from the organism and view the relation between the external fact and the mental impression as direct, we cannot always do so.
This being so, it is possible for errors of perception to arise through peculiarities of the nervous organization itself.

Thus, as I have just observed, sensibility has its limits, and these limits are the starting-point in a certain class of widely shared or _common_ illusions.

An example of this variety is the taking of the two points of a pair of compasses for one by the hand, already referred to.

Again, the condition of the nervous structures varies indefinitely, so that one and the same stimulus may, in the case of two individuals, or of the same individual at different times, produce widely unlike modes of sensation.
Such variations are clearly fitted to lead to gross _individual_ errors as to the external cause of the sensation.