THEORY

, a doctrine which terminates in the sole speculation or consideration of its object, without any view to the practice or application of it.

To be learned in an art, &c, the Theory is sufficient; to be a master of it, both the Theory and practice are requisite.—Machines often promise very well in Theory, but fail in the practice.

We say Theory of the moon, Theory of the rainbow, of the microscope, of the camera obscura, &c.

Theories of the Planets, &c, are systems or hypotheses, according to which the astronomers explain the reasons of the phenomena or appearances of them.