substance, in philosophy, term used to denote the changeless substratum presumed in some philosophies to be present in all being. Aristotle defined substance as that which possesses attributes but is itself the attribute of nothing. Less precise usage identifies substance with being and essence. The quest of philosophers for the ultimate identity of reality led some to define substance as one (see monism). Frequently the monist has identified substance with God, an absolute existing within itself and creating all other forms (Spinoza). According to dualism there are two kinds of substance. Descartes, for example, held that mind and matter constitute the two kinds of finite substance. Others have defined substance as material (Hobbes) or mental (Lotze), as static (Parmenides) or dynamic (Heraclitus), as knowable (Aristotle) or unknowable (Hume). Kant argued that our cognitive faculties require that we conceive of the world as containing substance, i.e., something that remains constant in the face of continuous change.

Chemical released by neurons to stimulate neighbouring neurons, allowing impulses to be passed from one cell to the next throughout the nervous system. A nerve impulse arriving at the axon terminal of one neuron stimulates release of a neurotransmitter, which crosses the microscopic gap (see synapse) in milliseconds to the adjoining neuron's dendrite. Many chemicals are believed to act as neurotransmitters. The few that have been identified include acetylcholine, dopamine, and serotonin. Some neurotransmitters activate neurons; others inhibit them. Some mind-altering drugs act by changing synaptic activity.

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