St. Thomas Aquinas [Summa totius logicæ Aristotelis (Opusculum 48)] divides the operations of the Understanding in reference to the logical character of their products into
Ratiocination, or Reasoning. [—]
Ratiocination or reasoning produces inferences or reasonings, which are expressed by argumentations, as, ” I think, therefore I must exist,” “Enoch, being a man, must have died; and since the Bible says he did not die, not everything in the Bible can be true.”
Ratiocination is defined by St. Thomas as the operation by which reason proceeds from the known to the unknown.
Reasoning is the process by which we attain a belief which we regard as the result of previous knowledge. [—]
Again, a given belief may be regarded as the effect of another given belief, without our seeming to see clearly why or how. Such a process is usually called an inference; but it ought not to be called a rational inference, or reasoning. A blind force constrains us. [—]
The word illation signifies a process of inference. Reasoning, in general, is sometimes called ratiocination. Argumentation is the expression of a reasoning.
Reasoning-power; or Ratiocination, called by some Dianoetic Reason, is the power of drawing inferences that tend toward the truth, when their premises or the virtual assertions from which they set out are true.