Thomas Aquinas ( c. 1225 to March 7, 1274), following Aristotle, would define man as a reasoning animal. The capacity to reason is a defining characteristic of man. It is only through our reason that we are able to distinguish truth from error.
We saw yesterday that light dwells within us as the life of man, the Divine Word. One aspect of this is reason and through reason we hope to attain wisdom which is the knowledge conveyed by the light received from the spirit we receive from God.
Aquinas points out "The prime author and mover of the universe is intelligence ..." He goes on to say "Blessed is the man that shall dwell in wisdom (Ecclus xiv, 22). The more sublime, because thereby man comes closest to the likeness of God, who hath made all things in wisdom (Ps. ciii, 24)."
In short than, human reason is a primary path to the clarity we require. In fact, human reason stands at a higher level than scripture in the sense that without reason we would be unable to judge what is scripture and what is not. The canon of scripture was set informally at first by those books and writings read in the assembly. As writings mulitiplied it became important to separate those that were authoritative from those that were not. This process continued through the 4th and early 5th centuries. We'll consider the canon closed by roughly the first decade of the 5th century, between 405 A.D. and the council of Carthage in 419 A.D. See this interesting discussion.
The canon was set through the action of reason on the documents received. Reason alone, taken in its modern sense as the operation of logical thought, was not enough. More properly reason is the whole aspect of human discernment which allows us to distinguish the truth from falsehood.
We have addressed to some degree the aspect of light but not yet the question of when we have illuminated matters enough. We'll work on that a little next.