skin. Objects nearly alike in size cannot be distinguished by common observation, and the error of observation may be measured as in the case of time and intensity. The correlation of the area of the stimulus with its intensity, and with the time of perception, may be determined through the error of observation. The perception of distance may be studied by similar methods, as, also, the perception of space by disparate senses. In the latter case apparently incommensurable sensations are correlated, somewhat as heat and mechanical work are correlated in physics.
21. 20. Mental complexity may be analogous to the configuration of a physical system.
We have seen that there are mental magnitudes corresponding to time, force, and space. But physical science is also concerned with the assemblage of relative positions of the parts of a material system, and the contents of the individual mind have at a given instant a certain complexity. Mental complexity and mental extensity (as their physical correlates) are closely related, the latter being perhaps an abstraction from the former. Whatever may be thought of these theoretical matters, certain experimental determinations must be acknowledged as valid. We can measure the amount which can be simultaneously perceived, – e.g., 5 disconnected letters, 15 letters when they make words, and 30 letters when the words make sentences. Complexes at first sight incommensurable are thus correlated. We can also determine the time or the number of repetitions needed to learn a complex too large to be perceived at once, and the rate at which such a complex fades from memory. The complexity of the contents of consciousness is correlated with its intensity and clearness. The less we attend to, the more exactly can we attend to it. In addition to the contents of consciousness distinctly given, there is a penumbra of mental processes indistinctly recognized, and beyond this, it may be, a vast range of sub-conscious mental processes, which would account for memory, association, etc. It can be shown by ex-
