By similar manipulations the simple needle may be made to feel like three or like four; now standing in a line, now in a triangle, and again in the corners of a square. But, since there is but one needle, what about the apparent distance _between_ these several points that are clearly _felt_? This is the most curious thing of all, and from the light it throws on the formation of our "ideas" both of number and of space, is the most important. To explain this: our notion of distance results out of "series" of sensations, in the same way as our notions of number. To have any idea of "distance" aroused between any two points of skin, the line of nerves lying between those points must, some time during life, have been previously stimulated in a line of succession, such as would result from a pencil drawn along between them. A card edge would give no idea of "distance" until such a series had some time been previously experienced. The memory of the "series" _is_ the idea of the distance. Within small areas of the skin, so few "series" have been experienced that no "distance memories" have been developed. Consequently pin-point areas commonly awaken no notion of distance. For some regions of the body these "limit areas" are larger than for others; at some places are quite large. On the back, spaces three inches apart may fail to give any idea of number or of distance. Every region has such a limit distance. _Now it is this limit distance, the smallest distance for which a "series" memory has been developed for a given region, that always shoves itself in, as the apparent distance between the several fictitious points felt from the single needle in our experiment. On the back the one needle feels like two set three inches apart; on the forehead like two half an inch apart; on the tongue one-sixteenth of an inch; and so on._ The upshot, then, of this matter is to show that our whole mind--our notions of space, number, time, and all else--is but a bundle of lawful habits, formed in relation with the things and occurrences around us. Ordinarily we have right ideas, because on the whole our mind has formed right habits. We have the right idea of an inch of skin, because the proper idea of an "inch long" has become habitually joined to each inch of skin, or in so far as this has been done. When a wrong idea gets joined, then we have an illusion; that is, the stretch of skin, or, as well, the pin-point of skin, seems a fraction of an inch in length; or, again, like three inches. "TIME REACTIONS:" METHODS OF MEASURING THE TIME REQUIRED FOR PERFORMING VARIOUS MENTAL ACTS. A sketch like this would be incomplete without a word about time reactions--a subject that historically was almost the first in the field, and has occupied more workers than any other. A generation ago "as quick as thought" was our extreme limit of expression. It outran "quicker than lightning." The great physiologist, Johannes Mueller, wrote, in 1844: