All things naturally tend to God implicitly, but not explicitly. That this may appear clearly it should be observed that a secondary cause can influence its effect only in so far as it receives the power of the first cause. The influence of an efficient cause is to act; that of a final cause is to be sought or desired. A secondary agent acts only by the efficacy of the first agent existing in it; similarly a secondary end is sought only by reason of the worth of the principal end existing in it inasmuch as it is subordinated to the principal end or has its likeness.
Accordingly, because God is the last end, He is sought in every end, just as, because He is the first efficient cause, He acts in every agent. But this is what tending to God implicitly, means. For the efficacy of the first cause is in the second as the principles of reasoning are in the conclusions. But to reduce conclusions to their principles or secondary causes to their first causes belongs only to the power of reasoning. Hence only a rational nature can trace secondary ends back to God by a sort of analytic procedure so as to seek God Himself explicitly. In demonstrative sciences a conclusion is correctly drawn only by a reduction to first principles. In the same way the appetite of a rational creature is correctly directed only by an explicit appetitive tendency to God, either actual or habitual.
Thomas Aquinas, Questions on Truth, Question 22, Article 2.