But man is not only a percipient being; he is also a being of will, and as such he comes into a relationship with the world which can be a source of rich experience. If one observes this relationship, one is bound to notice that it is based on the self-evident assumption that one possesses a lasting individuality, whose actions deal with a lasting material world. Any other way of behaviour would contradict the common sense of man; where we meet with it we are faced with a lunatic. Thus philosophy and common sense seem to stand in irreconcilable opposition to each other. But this opposition is only apparent. It exists so long as philosophy thinks it is able to come to valid conclusions without listening to the voice of common sense, believing itself to be too exalted to need to do so. Philosophy, then, does not realize 'that it has no other root but the principles of Common Sense; it grows out of them, and draws its nourishment from them: severed from this root, its honours wither, its sap is dried up, it dies and rots.' (I, 5.) At the moment when the philosophical consciousness ceases to regard itself as the sole foundation of its existence and recognizes that it can say nothing about itself without considering the source from which it has evolved, it attains the possibility of seeing the content of its experience in a new light. For it is no longer satisfied with considering this content in the completed form in which it presents itself. Rather does it feel impelled to investigate the process which gives rise to this content as an end-product (the 'impressions' and 'ideas' of Hume and his predecessors). Reid has faith in the fact - for his common sense assures him of it - that a lasting substantiality lies behind the world of the senses, even if for human consciousness it exists only so long as impressions of it are received via the bodily senses. Similarly, he has faith in the fact that his consciousness, although existing but intermittently, has as its bearer a lasting self. Instead of allowing this intuitively given knowledge to be shaken by a mere staring at fugitive pictures, behind which the real existence of self and world is hidden, he seeks instead in both directions for the origin of the pictures and will not rest until he has found the lasting causes of their transient appearances.