§ 3. =Mind and Body.=—The first thing to get clear about is the nature of the _man left in_ the world, the man whose presence is necessary for psychology and unnecessary for physics. Since we are talking science, this man will be man as science views him, and not the man of common sense; he will be, that is, the organism known to biology as _homo sapiens_, and not the self-centred person whom we meet in the everyday world of values. But the human organism owes its organic character, the organisation of its parts into a single whole, to its _nervous system_. All over the body and all through the body are dotted sense-organs, which take up physical and chemical impressions from their surroundings; these impressions are transmitted along nerve-fibres to the brain; in the brain they are grouped, arranged, supplemented, arrested, modified in all sorts of ways; and finally, it may be after radical transformation in the brain, they issue along other nerve-fibres to the muscles and glands. The nervous system thus receives, elaborates, and emits. Moreover, there is strong evidence to show that =the world which psychology explores= depends for its existence upon the functioning of the nervous system; or, if we prefer a stricter formula, that this world =is correlated with the functioning of the nervous system=. The _man left in_ thus reduces to a nervous system; and that is the truth of the statement, often met with in popular scientific writing, that the brain is the organ of mind. There is no organ of mind; that phrase is an echo of the old-world search after the place of residence of the mannikin-mind, which was assigned variously to heart, liver, eye, brain, blood, or was supposed somehow to perfuse the whole body. The scientific fact is that, whenever we come upon mental phenomena, then we also find a functional nervous system; we know nothing of the former apart from the latter; the two orders are thus correlated.