As Einstein has pointed out, general relativity does not account satisfactorily for the inertial properties of matter, so that an adequate theory of inertia is still lacking. This paper describes a theory of gravitation which ascribes inertia to an inductive effect of distant matter. In the rest-frame of any body the gravitational field of the universe as a whole cancels the gravitational field of local matter, so that in this frame the body is "free Thus in this theory inertial effects arise from the gravitational field of a moving universe. For simplicity, gravitational effects are calculated in flat space-time by means of Maxwell-type field equations, although a complete theory of inertia requires more complicated equations. This theory differs from general relativity principally in the following respects: (i) It enables the amount of matter in the universe to be estimated from a knowledge of the gravitational constant. (ii) The principle of equivalence is a consequence of the theory, not an initial axiom. (iii) It implies that gravitation must be attractive. The present theory is intended only as a model. A more complete, but necessarily more complicated theory will be described in another paper.