We proceed to the fourth article thus:
1. It seems that sacred doctrine is a practical science. For "the end of practical knowledge is action," according to the philosopher (2 Metaph., Text 3), and sacred doctrine is concerned with action, according to James 1:22: "Be ye doers of the word, and not hearers only." Sacred doctrine is therefore a practical science.
2. Again, sacred doctrine is divided into the Old and the New Law, and the Law has to do with the science of morals, which is practical. Sacred doctrine is therefore a practical science.
On the other hand: every practical science is concerned with the works of men. Ethics is concerned with their actions, and architecture with their buildings. But sacred doctrine is concerned principally with God, whose works men are. Hence it is not a practical science. Rather is it speculative.
I answer: as was said in the preceding article, sacred doctrine embraces matters dealt with by separate philosophical sciences while it itself remains one, because the formal nature to which it attends in diverse things is their being made known by the divine light. Hence even though some matters in the philosophical sciences are speculative and some practical, sacred doctrine includes them all within itself, just as God knows both himself and his works by the same knowledge. But sacred doctrine is more speculative than practical, since it is concerned with divine things more fundamentally than with the actions of men, in which it is interested in so far as through them men are brought to the perfect knowledge of God in which their eternal happiness consists. The answer to the objections is then obvious.