It will be seen that, according to our criterion of perception, an object perceived need not be in contact with the percipient’s body. The sun, moon, and stars are perceived according to the above criterion. In order, however, that an object not in contact with the body should be perceived, there are physical as well as physiological conditions to be fulfilled. There must be some physical process which takes place at the surface of the body when the object in question is suitably situated, but not otherwise; and there must be sense-organs capable of being affected by such a process. There are, as we know from physics, many processes which fulfil the necessary physical conditions, but fail to affect us through the inadequacy of our sense-organs. Waves of a certain sort make sound, but waves of exactly the same sort become inaudible if they are too short. Waves of a certain sort make light, but if they are too long or too short they are invisible. The waves used in wireless are of the same sort as those that make light, but are too long. There is no reason _a priori_ why we should not be aware of wireless messages through our senses, without the need of instruments. X-rays are also of the same sort as those that make light, but in this case they are too short to be seen. They might render the objects from which they come visible, if we had a different sort of eye. We are not sensitive to magnetism, unless it is enormously powerful; but if we had more iron in our bodies, we might have no need of the mariner’s compass. Our senses are a haphazard selection of those that the nature of physical processes renders possible; one may suppose that they have resulted from chance variation and the struggle for existence. It is important to observe that our perceptions are very largely concerned with form or shape or structure. This is the point emphasised by what is called “_Gestaltpsychologie_”, or psychology of form. Reading is a case in point. Whether we read black letters on white paper or white letters on a blackboard is a matter which we hardly notice; it is the forms of the letters that affect us, not their colour or their size (so long as they remain legible). In this matter, the sense of sight is pre-eminent, although blind men (and others to a less degree) can acquire a good knowledge of form by the sense of touch.