19 If in our discussion of the nature and birth of God we adduce certain analogies, let no one suppose that such comparisons are perfect and complete. There can be no comparison between God and earthly things, yet the weakness of our understanding forces us to seek for illustrations from a lower sphere to explain our meaning about loftier themes. The course of daily life shows how our experience in ordinary matters enables us to form conclusions on unfamiliar subjects. We must therefore regard any comparison as helpful to man rather than as descriptive of God, since it suggests, rather than exhausts, the sense we seek. Nor let such a comparison be thought too bold when it sets side by side carnal and spiritual natures, things invisible and things palpable, since it avows itself a necessary aid to the weakness of the human mind, and deprecates the condemnation due to an imperfect analogy. On this principle I proceed with my task, intending to use the terms supplied by God, yet colouring my argument with illustrations drawn from human life.
Source: On the Trinity (New Advent)