When a man endeavours to discover what is passing in the mind of another, by reflecting on what has passed or is passing in his own, the certainty of the result depends of course on the degree in which the two minds coincide. The educated man, therefore, estimates ill the feelings and the faculties of the uneducated, the adult those of the child, the sane those of the insane, the civilised man those of the savage. And this accounts for the constant mismanagement of the lower orders, and of children, madmen, and savages, by their intellectual and moral superiors. The student of mental science is in the situation of an anatomist, allowed to dissect only a single subject, and forced to conjecture the internal conformation of other men by assuming that it resembles that of the subject which he has dissected, and correcting that assumption only by observing the forms of their bones and the outward play of their muscles. The mental peculiarities of other men are likely to mislead him in particular instances. His own mental peculiarities are likely to mislead him on all occasions. Another important difference, between mental and physical studies, is the degree and the manner in which they respectively can be aided by experiment. When we are dealing with matter, we frequently are able to combine its particles at will, and to ascertain the results of the combination. If we find that, all other things remaining the same, the presence or absence of a given element is followed by the presence or absence of a given result, we ascribe to that element and to that result the relation of cause and effect, or at least of condition and result. But we can scarcely be said to be able to make experiments on the minds of others. It is necessary to an experiment, that the observer should know accurately the state of the thing observed before the experiment, and its state immediately after it. But when the minds of other men are the subject, we can know but little of either the one state or of the other. We are forced, therefore, to rely not on experiment, but on experience, that is to say, not on combinations of known elements effected for the purpose of testing the result of each different combination, but on our observation of actual occurrences, the results of the combination of numerous elements, only a few of which are within our own knowledge. And the consequence is, that we frequently connect facts which are really independent of one another, and not unfrequently mistake obstacles for causes.