Fruits of Philosophy: A Treatise on the Population Question

Consciousness is not a "principle" or substance of any kind, nor is it, strictly speaking, a property of any substance or being. It is a peculiar action of the nervous system, and the system is said to be sensible, or to possess the property of sensibility, because those sentient actions which constitute our different consciousnesses may be excited in it. The nervous system includes not only the brain and spinal marrow, but numerous soft white cords, called nerves, which extend from the brain and spinal marrow to every part of the body in which a sensation can be excited.
A sensation is a sentient action of a nerve and the brain; a thought or idea (both the same thing) is a sentient action of the brain alone. A sensation or a thought is consciousness, and there is no consciousness but that which consists either in a sensation or a thought.
Agreeable consciousness constitutes what we call happiness, and disagreeable consciousness constitutes misery. As sensations are a higher degree of consciousness than mere thought, it follows that agreeable sensations constitute a more exquisite happiness than agreeable thoughts. That portion of happiness which consists in agreeable sensations is commonly called pleasure. No thoughts are agreeable except those which were originally excited by or have been associated with agreeable sensations. Hence, if a person never had experienced any agreeable sensations, he could have no agreeable thoughts, and would, of course, be an entire stranger to happiness.
There are five species of sensations—seeing, hearing, smelling, tasting and feeling. There are many varieties of feeling—as the feelings of hunger, thirst, cold, hardness, etc. Many of these feelings are excited by agents that act upon the exterior of the body, such as solid substances of every kind, heat, and various chemical irritants. These latter feelings are called passions.
Those passions which owe their existence chiefly to the state of the brain, or to causes acting directly upon the brain, are called the moral passion. They are grief, anger, love, etc. They consist of sentient actions, which commence in the brain and extend to the nerves in the region of the stomach, etc. But when the cause of the internal feeling of passion is seated in some organ remote from the brain, as in the stomach, genital organs, etc., the sentient action which constitutes the passion commences in the nerves of such organ and extends to the brain, and the passion is called an appetite, instinct, or desire. Some of these passions are natural, as hunger, thirst, the reproductive instinct, the desire to urinate, etc. Others are gradually acquired by habit A hankering for stimulants, as spirits, opium and tobacco, is one of these.
Such is the nature of things that our most vivid and agreeable sensations cannot be excited under all circumstances, nor beyond a certain extent under any circumstances, without giving rise in one way or another to an amount of disagreeable consciousness or misery, exceeding the amount of agreeable consciousness which attends such ill-timed or excessive gratification. To excite agreeable sensations to a degree not exceeding this certain extent is temperance; to excite them beyond this extent is intemperance; not to excite them at all is mortification or abstinence. This certain extent varies with different individuals, according to their several circumstances, so that what would be temperance in one person may be intemperance in another.