To return now to the subject of 'logic-tight compartments'. Each contains a collection of ideas which are treated by the owner in a special way, cherished and guarded carefully from those forces which may cause modification. At the same time he will probably refuse to admit that they influence his consideration of certain questions related to them. The more logic-tight the compartment is, the more warmly does its owner defend it; but where plain reasoning is concerned few men can be roused to enthusiasm. Even though there may be people who regard the reasonings of Euclid as purely appeals to the emotions, what mathematician could grow excited about a man who denied the truth of the Fifth Proposition? But to run counter to a man's political or social beliefs is a sure way to raise the controversial temperature. As will be easily seen, rationalisation is of everyday occurrence with all of us, and the man who rationalises always believes he is reasoning. Consider now the business rogue who makes a success of his roguery and then launches out as a philanthropist, still continuing his roguery as a permanent side-line. Such cases are not unknown, and the man seems able to carry on without any sense of conflict between his two activities. Or consider those not uncommon instances where a man prominent in religious work is detected in some financial crime; it is usual to regard him as a hypocrite who has used religion as a cloak, but it is equally probable that he was honestly religious, that his earliest steps into crime were reconciled to his principles by rationalisations, and, as he advanced, a logic-tight compartment was built up to prevent conflict between his wrong-doing and his self-respect. In these examples we have a part of the stream which comes into contact with the main stream of consciousness only by means of a process of rationalisation which allows the two to exist without great mental conflict, but this will never be admitted by the owner, though other people may be acutely conscious of it. Here, to simplify explanation, I must introduce the word _complex_ as used to indicate a system of ideas having a common centre,[2] whether the system is present in the consciousness or exists only in the unconscious. [Footnote 2: The word 'complex' was originally used by Freud only in regard to ideas existing in the unconscious, but the way in which I use it is convenient and follows the custom of some English writers.]