In one direction Reid finds himself led to the outer boundary of the body, where sense perception has its origin. This prompts him to investigate the perceptions of the five known senses: smelling, tasting, hearing, touching and seeing, which he discusses in this order. In the other direction he finds himself led - and here we meet with a special attribute of Reid's whole philosophical outlook - to the realm of human speech. For speech depends upon an inner, intelligent human activity, which, once learnt, becomes a lasting part of man's being, quite outside the realm of his philosophizing consciousness, and yet forming an indispensable instrument for this consciousness.