Objection 1. It would seem that the process of counsel is indefinite. For counsel is an inquiry about the particular things with which action is concerned. But singulars are infinite. Therefore the process of counsel is indefinite.
Objection 2. Further, the inquiry of counsel has to consider not only what is to be done, but how to avoid obstacles. But every human action can be hindered, and an obstacle can be removed by some human reason. Therefore the inquiry about removing obstacles can go on indefinitely.
Objection 3. Further, the inquiry of demonstrative science does not go on indefinitely, because one can come to principles that are self-evident, which are absolutely certain. But such like certainty is not to be had in contingent singulars, which are variable and uncertain. Therefore the inquiry of counsel goes on indefinitely.
On the contrary, "No one is moved to that which he cannot possibly reach" (De Coelo i, 7). But it is impossible to pass through the infinite. If therefore the inquiry of counsel is infinite, no one would begin to take counsel. Which is clearly untrue.
I answer that, The inquiry of counsel is actually finite on both sides, on that of its principle and on that of its term. For a twofold principle is available in the inquiry of counsel. One is proper to it, and belongs to the very genus of things pertaining to operation: this is the end which is not the matter of counsel, but is taken for granted as its principle, as stated above (Article 2). The other principle is taken from another genus, so to speak; thus in demonstrative sciences one science postulates certain things from another, without inquiring into them. Now these principles which are taken for granted in the inquiry of counsel are any facts received through the senses--for instance, that this is bread or iron: and also any general statements known either through speculative or through practical science; for instance, that adultery is forbidden by God, or that man cannot live without suitable nourishment. Of such things counsel makes no inquiry. But the term of inquiry is that which we are able to do at once. For just as the end is considered in the light of a principle, so the means are considered in the light of a conclusion. Wherefore that which presents itself as to be done first, holds the position of an ultimate conclusion whereat the inquiry comes to an end. Nothing however prevents counsel from being infinite potentially, for as much as an infinite number of things may present themselves to be inquired into by means of counsel.
Reply to Objection 1. Singulars are infinite; not actually, but only potentially.
Reply to Objection 2. Although human action can be hindered, the hindrance is not always at hand. Consequently it is not always necessary to take counsel about removing the obstacle.
Reply to Objection 3. In contingent singulars, something may be taken for certain, not simply, indeed, but for the time being, and as far as it concerns the work to be done. Thus that Socrates is sitting is not a necessary statement; but that he is sitting, as long as he continues to sit, is necessary; and this can be taken for a certain fact.
Source: Summa Theologica (New Advent)