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the belief had established itself that mental and nervous disorders were due to deranged bodily processes, and in particular to diseases of the brain. Investigation was then naturally directed to determining precisely what bodily processes were at fault, and treatment to remedies by which these processes could once more be made normal. This "physiological conception," as we may call it, still holds its place, but its sway is no longer undisputed, because during the past fifty years yet another conception has come into being. In this period psychology has freed itself from the metaphysics, ethics and other alien companions which formerly hampered its progress, and has developed into a positive science. As a result of this development we have learnt, not that events may have mental causes, because that we always knew, but that these mental causes are capable of exact scientific estimation, that they act according to precise scientific laws, and that they are capable of being influenced or removed by scientific methods of treatment. With this recognition of the fact that mental events follow one another in a rigidly determined chain of cause and effect there has come a new "psychological conception" of mental and nervous disorders, which holds that some at least of their phenomena are due to mental causes, capable of determination by psychological investigation, and of removal by psychological methods of treatment.
It will perhaps be helpful to make clear by some simple examples precisely what is meant by "mental" and "physical" causes. Weeping may be produced
