Synechism (from Greek συνεχής synechḗs, "continuous" + -ism, from σύν syn, "together" + ἔχειν échein>, "to have", "to hold") is a philosophical term proposed by C. S. Peirce to express the tendency to regard things such as space, time, and law as continuous:

The things of this world, that seem so transitory to philosophers, are not continuous. They are composed of discrete atoms, no doubt Boscovichian points. The really continuous things, Space, and Time, and Law, are eternal.

His synechism holds that the essential feature in philosophic speculation is continuity. It denies that all is merely ideas, likewise that all is merely matter, and mind–matter dualism.

The adjective "synechological" is used in the same general sense; "synechology" is a theory of continuity or universal causation; "synechia" is a term in ophthalmology for a morbid union of parts.

Peirce held that there are three elements or categories throughout experience:

Peirce held that firstness and secondness, as elements, give thirdness and continuity something upon which to operate, and that continuity governs all experience and every element in it.

Synechism is specially directed to the question of hypothesis, and holds that a hypothesis is justifiable only on the ground that it provides an explanation. All understanding of facts consists in generalizing concerning them. Generalization is seen as movement by thought toward continuity. The fact that some things are ultimate may be recognized by the synechist without abandoning his standpoint, since synechism is a normative or regulative principle, not a theory of existence.

In a contribution to an 1887 symposium Science and Immortality, Peirce had said that the question stood undecided on whether there is immortality or at least "a future life", but also that science could come to shed light on the question.