Two conditions determining the construction of a body of scientific theory and imposing limitations on the choice of a language to embody that theory are that the language should correlate a variety of experimental results and that it should lead to expectations which can be confirmed or falsified by later experiments. This leads to the problem of a choice between using terms already current in ordinary speech, redefining them to give them technical precision, and of constructing a purely technical vocabulary, the terms of which we both create and define. 'Repression', for example, derives from popular language whereas 'cathexis' is a specific technical term. The danger of using terms from current ordinary speech is that they create an illusion of understanding, particularly