Before providing an answer to this question, it is important to clarify a term. According to the Merriam-Webster Dictionary, "base" when used as an adjective in this context means "lacking or indicating the lack of higher qualities of mind or spirit; lacking higher values." So the term "baser impulses" would mean impulses that lack morality or are inferior mentally or spiritually. However, this term does not fit the example that Benjamin Franklin gives in The Autobiography of Benjamin Franklin.

In Franklin's example, he talks about "not eating animal food," which means he was resolved to be either vegetarian or vegan. Vegans not only avoid eating meat in any form, but also don't eat any eggs, dairy, or other food from animal sources. He initially justifies his food choices by stating that the killing of fish is murder. However, he modifies his stance when he realizes that fish kill other fish for food. He uses this as a rationalization that it should be fine for humans to also use fish for food. As a result of his cogitations, which are valid, he adjusts his stance from being vegetarian to being pescatarian, which means he allows the eating of fish and other seafood in his otherwise vegetarian diet.

From this example alone, we reason that his final statement, which states that reason "enables one to find or make a reason for everything one has a mind to do," can be an exaggeration. The example he uses is in fact a matter of opinion, although fervent vegans and vegetarians might argue otherwise. Some people eat meat and others don't; there are many arguments to support both sides, and both viewpoints are accepted in our society now, and they were in Franklin's time as well. His statement that people can find a reason for anything that they might think to do is an exaggeration, as it does not stand up, for instance, if someone is contemplating murder. No matter what reasoning goes through a person's mind, the deed is wrong.

In conclusion, in Franklin's example, he is not justifying a "baser impulse," but rather simply explaining the reasoning that caused him to change his opinion on a debatable topic. However, his comment at the end does not always hold up, as it is not always possible to justify "everything one has a mind to do" through reason.

Benjamin Franklin here makes himself the butt of his own joke in two different ways. First, he seeks a justification for eating fish, behavior that few of his audience would have bothered to excuse at all, since vegetarianism was such a rarity in the eighteenth century. Then, when he chooses a justification, it is a preposterous version of the "appeal to nature" fallacy. This would have sounded even more hollow to Franklin's first readers than it does to us, since even those who were not religious would have considered humanity unique and entirely distinct from the animal kingdom. Franklin's thought that "if you eat one another, I don't see why we mayn't eat you" is therefore a flippant justification, verging on the blasphemous, for something which requires no justification in the first place.

This anecdote is therefore a very effective illustration of the general principle that human beings are rationalizing creatures rather than rational ones. Instead of being motivated by reason, we do what we want, then use reason after the fact to justify our conduct. Franklin, every inch a son of the Enlightenment, clearly regrets this and wants us all to be more genuinely reasonable, including himself. However, his self-deprecating humor in using himself as the example of how not to use reason is more effective in demonstrating the validity of his point than hectoring the reader or being too obviously didactic. Few readers will be able to examine their own consciences and conclude that they have never done the same as the author does here, making the truth of his point self-evident without laboring it.

Ben Franklin had a sense of humor, and it was not unusual for him to comment on his observations in a tongue-in-cheek sort of manner. In discussing the positive aspects of spending romantic time with older women, Franklin once several reasons, ending with this, that "They are so grateful."  The story about the fish demonstrates Franklin's understanding of human nature, and his ability to recognize when he is rationalizing something he wants to do but has said he would not. I suspect that Franklin would be amused by the compulsive spender standing in a store explaining to anyone who will listen why he or she needs, not wants, but needs the item he or she is getting ready to put on the Visa card.  Franklin was a master of observing human nature, as recorded in his many quotes, comments and essays.  Although he was willing to laugh at himself, he did take self-improvement very seriously, one year creating a list of virtue that he would practice week by week.  The original Franklin Quest day planner system, which is now part of the Franklin Covey company, is based on Ben Franklin's self-improvement notebooks and refers to the virtues he identified and practiced.