So far as an impulse owes its character to its stimulus (or to such effects of past accompanying or connected stimuli as are revived) so far is it a _reference_, to use the term which we introduced in Chapter XI, to stand for the property of mental events which we substitute for thought or cognition.[95] It is plain that the independent internal conditions of the organism usually intervene to distort reference in some degree. But very many of our needs can only be satisfied if the impulses are left undistorted. Bitter experience has taught us to leave some of them alone, to let them reflect or correspond with external states of affairs as much as they can, undisturbed as far as possible by internal states of affairs, our needs and desires. In all our behaviour can be distinguished stimuli we receive, and the ways in which we use them. What we receive may be any kind of stimulus, but only when the reaction we make to it tallies with its nature and varies with it in quasi-independence of the uses we make of it does reference occur. Those to whom visual images are of service in considering complex matters may find it convenient at this point to imagine a circle or sphere constantly bombarded by minute particles (stimuli). Within the sphere may be pictured complex mechanisms continually changing for reasons having nothing to do with the external stimuli. These mechanisms by opening little gateways select which of the stimuli shall be allowed to come in and take effect. So far as the subsequent convulsions are due to the nature of the impacts and to lingering effects of impacts which have accompanied similar impacts in the past, the convulsions are referential. So far as they are due to the independent motions of the internal mechanisms themselves, reference fails. This diagrammatic image may possibly be of convenience to some. By those who distrust such things it may with advantage be disregarded. It is not introduced as a contribution to neurology, and is in no way a ground for the author’s view. The extent to which reference is interfered with by needs and desires is underestimated even by those who, not having yet forgotten the events of 1914-1918, are most sceptical as to the independence of opinions and desires. Even the most ordinary and familiar objects are perceived as it pleases us to perceive them rather than as they are, whenever error does not directly deprive us of advantages. It is almost impossible for anyone to secure a correct impression of his own personal appearance or of the features of anyone in whom he is personally interested. Nor is it perhaps often desirable that he should.