Ironically, this challenge was laid to rest over two centuries ago by the philosopher often most associated with extreme skepticism, David Hume:
For here is the chief and most confounding objection to excessive scepticism, that no durable good can ever result from it; while it remains in its full force and vigour. We need only ask such a sceptic, What his meaning is? And what he proposes by all these curious researches? He is immediately at a loss, and knows not what to answer. A Copernican or Ptolemaic, who supports each his different system of astronomy, may hope to produce a conviction, which will remain constant and durable, with his audience. A Stoic or Epicurean displays principles, which may not be durable, but which have an effect on conduct and behaviour. But a Pyrrhonian cannot expect, that his philosophy will have any constant influence on the mind: or if it had, that its influence would be beneficial to society. On the contrary, he must acknowledge, if he will acknowledge anything, that all human life must perish, were his principles universally and steadily to prevail. All discourse, all action would immediately cease; and men remain in a total lethargy, till the necessities of nature, unsatisfied, put an end to their miserable existence. It is true; so fatal an event is very little to be dreaded. Nature is always too strong for principle. And though a Pyrrhonian may throw himself or others into a momentary amazement and confusion by his profound reasonings; the first and most trivial event in life will put to flight all his doubts and scruples, and leave him the same, in every point of action and speculation, with the philosophers of every other sect, or with those who never concerned themselves in any philosophical researches. When he awakes from his dream, he will be the first to join in the laugh against himself, and to confess, that all his objections are mere amusement, and can have no other tendency than to show the whimsical condition of mankind, who must act and reason and believe; though they are not able, by their most diligent enquiry, to satisfy themselves concerning the foundation of these operations, or to remove the objections, which may be raised against them.
Santayana expanded on this theme in his Scepticism and Animal Faith, which is one of the most thorough critiques of foundationalism to date. Pursing doubt to its ultimate end, Santayana challenges self-consciousness, discourse, logic, change, memory and time. In doing so, he goes well beyond Descartes’ doubts to discover the ultimate certainty, the perusal of a passing datum, a mere instance of awareness. This “solipsism of the present moment,” Santayana concludes, cannot possibly be a bedrock of certainty, because it does not constitute knowledge. It is the awareness of a given without a basis for belief. Knowledge does not arise until intelligence arrives on the scene and connects these instances of awareness into larger, meaningful wholes, which can then be interpreted as symbols of a posited, external reality existing in time and space. Since this exercise of intelligence is not given, why then should it be trusted? Santayana answers “by animal presumption, positing whatever object instinct is materially predisposed to cope with, as in hunger, love, fighting, or the expectation of the future.” In other words, it is by an instinct, which Santayana calls “animal faith.” But this animal faith is by no means an entirely groundless or “arbitrary” inclination, but one which is tested and corroborated during every moment when intelligence holds dominion over our lives.
Critical to Santayana’s view is the notion that some views are biologically inevitable, so that philosophers who deny them are not being altogether sincere. “I should be ashamed to countenance opinions which, when not arguing, I did not believe,” he insists. Some beliefs are inevitable because they have been bred in us by evolution (or by “nature,” if you prefer). These are beliefs that are associated with successful action, which is often the best test of truth. For Santayana, while the nature of truth is correspondence, the test of truth is pragmatic. This is a different orientation than what is found in Objectivism, which, in relation to these issues, often equivocates between rationalistic speculation (e.g., the Objectivist axioms) and an extreme empiricism (e.g., basing all knowledge on the “evidence” of the senses). But the ultimate raison d’être of knowledge is to cope with animal needs; and so whatever knowledge best satisfies these needs, which leads to successful action and solves the most problems in the real world, is that knowledge which most likely has the stamp of truth about it.
Santayana concludes as follows:
Living when human faith is again in a state of dissolution, I have imitated the Greek sceptics in calling doubtful everything that, in spite of common sense, any one can possibly doubt. But since life and even discussion forces me to break away from a complete scepticism, I have determined not to do so surreptitiously nor at random, ignominiously taking cover now behind one prejudice and now behind another. Instead, I have frankly taken nature by the hand, accepting as a rule in my farthest speculations the animal faith I live by from day to day. There are many opinions which, though questionable, are inevitable to a thought attentive to appearance, and honestly expressive of action. These natural opinions are not miscellaneous, such as those which the Sophists embraced in disputation. They are superposed in a biological order, the stratification of the life of reason. In rising out of passive intuition, I pass, by a vital constitutional necessity, to belief in discourse, in experience, in substance, in truth, and in spirit. All these objects may conceivably be illusory. Belief in them, however, is not grounded on a prior probability, but all judgements of probability are grounded on them. They express a rational instinct or instinctive reason, the waxing faith of an animal living in a world which he can observe and sometimes remodel.