Monad (Gr. , unity), in philosophy, a word used by the Neo-Platonists of the early ages of Christianity, and especially by Origen, to express an idea of Divinity, and also the union of the Divine Spirit with matter. According to them, the soul was created before all other beings, and, being made divine by the knowledge of the monad, became Christ; a doctrine which, according to some writers, is the basis of Arianism. Long after this the word was used by Leibnitz to designate the primordial elements of all matter. According to him, monads are material points, possessing different degrees of consciousness and intelligence. The monad is simple, without extent, incorruptible, and so constituted that its whole future is contained in its beginning. (See Leibnitz.) - In the new chemistry the monatomic elements, such as hydrogen, chlorine, and potassium, whose molecules are capable of uniting only with single molecules of other elements, are called monads; while other elementary molecules, from their capacity to unite with two or three more molecules, are called diads, triads, etc. (See Atomic Tiieoey). - A number of infusorial organisms have received the name of monads.
Some of these manifestly belong to the animal kingdom, some to the vegetable, while of others it is difficult to say to which kingdom they belong. The first classification was by the Danish naturalist O. F. Mtiller, who arranged under the same genus (rnonas) the mere moving specks that are developed in infusions, whether in vessels allowed to stand, or placed between slips of glass under the microscope, and also certain of the most elementary and smaller of the ciliated infusoria, of which the monas lens is the most abundant representative. Ehrenberg in a subsequent classification placed certain organisms containing cells in their interior among poly-gastric infusoria, and called them monads. More recent observers regard them as belonging to the vegetable kingdom, ranking them among the algae. The development of the monas lens from bacteria, and their subsequent transformation into amoebae, and finally into bacteria, is a subject of rare interest, which has been pursued by Haeckel, Pineau, Pouchet, Bas-tian, and others. (See Animalcules, Infusoria, Protoplasm, and Spontaneous Generation).