  Reid's treatment of arguments for God's existence is for the most part unoriginal, and freely makes use of ideas of many previous Christian thinkers along this well-worn trail. Reid's cosmological argument stems from Samuel Clarke's and features a priori justification of the principle, “That whatever begins to exist, must have a cause which produced it” (EIP 6.6, 497). Reid defends this against a few attacks associated with Hume. For example, if Hume's skepticism about causation undermines this principle, Reid says that many everyday inferences would become unjustified (EIP 6.6, 497–8). Reid uses this principle in a cosmological argument, but only as recorded in his lecture notes (Natural Theology 66–7), which leaves much to be desired. Every being must be either contingent or necessary. “We call that contingent which either might or might not be and that necessary which must be. Whatever either might or might not be depends on the will of some agent with power to bring it to pass or not...” (Natural Theology 66). Reid is then recorded to argue that to suggest that the “Supreme Being” exists contingently “evidently would be absurd” (Natural Theology 66). He does not here discuss the unintelligibility of a one-directional infinite series of contingent causes or of a circular series of contingent causes.