system. This selective power is like that of a bone, muscle, or nerve, each of which will only take to itself from the blood its own proper ingredients. Brewster observed that 'In the complex formation of apophyllite and analcime, laws operate more like those in living structures than in crystalline formations.' The property possessed by substances in general of enabling the application of one force to produce a number of effects simultaneously, may also be regarded as additional evidence of the great complexity of material substances.

1

We may conclude from these and many other similar facts, that we are surrounded on the one hand by phenomena of almost infinite magnitude, and on the other by an endless number of others of almost infinite minuteness and complexity.

CHAPTER V.

ON IDEAS.

Man is a thinking being, whether he will or no; all he can do is to turn his thoughts the best way.-SIR W. TEMPLE.

THE mind operates in scientific research on perceptions or ideas. An idea is also a mental impression; and, if we adopt the theory of unconscious cerebration,' it may or may not be attended by consciousness and perception. It is produced in the cerebrum by nervous force, which is set in motion by various external and internal causes, by external objects or forces acting through the senses, by physical or

1 Philosophical Magazine, vol. v., 1853, pp. 17-27.

mental excitement of the brain, by memory and associative suggestion, &c.; but how it is produced we do not yet know. Frequently, when mental action is strong, the head becomes suddenly hot and the feet cold. The seat of perception and ideas is believed to be in the grey cortical nervous matter of the convolutions of the cerebrum. By the study of physiology it has been placed beyond doubt that the nerve-cells, which exist in countless numberabout 600,000,000 according to Meynert's calculationsin the grey matter spread over the surface of the hemispheres, are the nervous centres of ideas.' All mental action appears to depend upon and to produce physical cerebral impressions, and sensations originally precede ideas.

6

According to the doctrine of relativity,' 2 we only feel or perceive a change of state; a thought includes a perception of relation of similarity or difference. The degree of conscious impression made upon our senses or perceptive powers depends upon their immediately previous state. Cold water feels more cold to a hand which has been previously warmed than to one already cool, because in the former case there is a greater degree of nervous change. The greater and more sudden also the degree of nervous or mental alteration, the stronger the sensation or mental impression; and we are only conscious of the stronger and more sudden sensorial and cerebral changes, because our senses and perceptive powers are not sufficiently refined to enable us to feel or perceive the more gradual or more minute ones. It is the most conspicuous differences which most impress us. The term consciousness is usually taken to mean sensibility in general; all our primary con

1 Maudsley, Physiology of Mind, p. 259.

2 See Bain, Senses and Intellect, 2nd edit. p. 9.

CONSCIOUSNESS DEPENDENT UPON CHANGE.

37

sciousness is in ourselves, and is only referred to external causes by the aid of the intellect.

A thing cannot act alone, or upon itself; perfect sameness is inert, it cannot move. To produce consciousness there must be a difference, a thing acting and a thing acted upon the latter being the human brain. A photographic plate cannot take an image of itself, neither can the cerebral substance which perceives perceive itself -the two actions are simultaneously incompatible; we cannot think, and at the same time think of that act of thought. Perfect continuity, sameness, or non-variation of cause, has no effect upon our perceptive powers, and therefore we are unable to perceive directly time, space, force, or motion in themselves; we feel not the uniform pressure of the atmosphere, nor the motion of the earth. The dependence of consciousness upon change of impression is largely proved by the fact, that whilst the mind is highly incapable of completely realising ultimate ideas; or those of the great static uniformities of space, time, and infinite potential power; or the great cause of all things—it is quite capable of perceiving those of sequences or of orders of succession of mental impressions, because in the latter case only is there great and rapid mental change. Also, although we can but little conceive ideas of the essential natures of the modes of energy which produce physical and chemical effects, we can very much more completely realise the order of effects which those forces produce; and our conceptions of ultimate power, of causation, and of the relations of cause and effect,

depend upon this ability.

There are many things which we neither know nor can know in themselves, that is, in their direct and immediate relation to our faculties of knowledge, but which manifest themselves through the medium of their effects. Consciousness cannot exist in

38

GENERAL VIEW AND BASIS OF SCIENTIFIC RESEARCH.

dependently of some peculiar modification of the mind; we are only conscious as we are conscious of a determinate state. To be conscious we must be conscious of some particular perception, remembrance, imagination, or feeling; we have no general consciousness.' Persistence of ideas in consciousness is the basis of our knowledge of realities, and is also to a certain extent a test of truth.

1

Consciousness enables us to define many things, but not itself. What is consciousness? is about the most difficult of all questions for man to answer, because it is asking self what is self, and the answer given can only be a repetition-consciousness is consciousness. Consciousness appears to be a power of perceiving mental changes, whether resulting from sensation or volition; and arises from a sufficiently strong and rapid nervous or mental change. It includes both sensation and perception, and the total consciousness in both these forms constitutes the human I or Ego. Consciousness therefore differs in kind: there is physical, or that of sense; and mental, or that of mind-and the latter is the more complex. Mental consciousness is not mind, nor co-extensive with it; but only a variable accompaniment of it. As there may be physical activity without physical consciousness, so may there he mental activity without mental consciousness, but not the reverse—we often catch ourselves thinking. There is no abstract consciousness. Consciousness differs also in degree, from that which accompanies feeble sensations and ideas to that concomitant with the most excited action of all our senses and mental powers. The consciousness attending one action often excludes that of another; if the consciousness of feeling is stronger than that of the

1 Sir W. Hamilton, Lectures on Metaphysics, vol. i. p. 348; Winslow, On Obscure Diseases of the Brain and Mind, p. 437.

CONSCIOUSNESS A RESULT OF PERSONAL ACTIVITY. 39

intellect, it shuts the latter out; but if, on the other hand, that of the intellect is strongest, the intellect prevails.

As consciousness is a result of a sufficiently high degree of mental or physical activity, unconsciousness and sleep are promoted by absence of all conditions which excite the mind. Our various powers cease to induce consciousness, usually in something like the following order :-By absence of physical pain, uneasiness, or excitement in any part of our body, organic sensation ceases and no longer excites the mind. By perfect stillness of limbs, the sense of touch ceases in a similar manner to arouse perception. By absence of flavours, odours, sound, and light, the senses of taste, smell, hearing, and sight become quiescent, and memory alone remains as a source of mental excitement; and by persistent exclusion of the more exciting ideas only, memory also becomes unconscious. As, however, by withdrawing the mental perception from one class of ideas it is thereby better enabled to be concentrated upon others, quiescence of all the senses is a favourable condition for conscious thought and reflection. One of the most effectual means of preventing this is previous cheerful conversation or other agreeable occupation, which, by dispelling anxious thoughts and discharging outwardly the nervous power, promotes sleep.

The production and existence of ideas are results of our capacity of receiving sensorial and cerebral impressions. The degree of our sensitiveness to particular impressions and ideas depends upon that which is born in us and that which is subsequently acquired; and it is generally considered that in all cases of genius and extraordinary mental ability of any kind, a high degree of inherited tendency to receive a particular class of mental impressions exists.