Moreover, as soon as the child has learned to use correctly relational words, such as “eat”, he has become capable of being causally affected by a relational feature of the environment, which involves a new degree of complexity not required for the use of ordinary nouns. Thus the correct use of relational words, _i.e._ of sentences, involves what may be correctly termed “perception of form”, _i.e._ it involves a definite reaction to a stimulus which is a form. Suppose, for example, that a child has learnt to say that one thing is “above” another when this is in fact the case. The stimulus to the use of the word “above” is a relational feature of the environment, and we may say that this feature is “perceived” since it produces a definite reaction. It may be said that the relation _above_ is not very like the word “above”. That is true; but the same is true of ordinary physical objects. A stone, according to the physicists, is not at all like what we see when we look at it, and yet we may be correctly said to “perceive” it. This, however, is to anticipate. The definite point which has emerged is that, when a person can use sentences correctly, that is a proof of sensitiveness to formal or relational stimuli. The structure of a sentence asserting some relational fact, such as “this is above that”, or “Brutus killed Cæsar”, differs in an important respect from the structure of the fact which it asserts. _Above_ is a relation which holds between the two terms “this” and “that”; but the _word_ “above” is not a relation. In the sentence the relation is the temporal order of the words (or the spatial order, if they are written), but the word for the relation is itself as substantial as the other words. In inflected languages, such as Latin, the order of the words is not necessary to show the “sense” of the relation; but in uninflected languages this is the only way of distinguishing between “Brutus killed Cæsar” and “Cæsar killed Brutus”. Words are physical phenomena, having spatial and temporal relations; we make use of these relations in our verbal symbolisation of other relations, chiefly to show the “sense” of the relation, _i.e._ whether it goes from A to B or from B to A.