In Part One of "The Dialogues on Natural Religion" by David Hume, Cleanthes accuses Philo of attempting to erect religious faith on philosophical skepticism. What does Cleanthes mean by doing this?

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In Part One, Cleanthes means to expose what he believes are flaws in philosophical skepticism. Philosophical skepticism is the belief that human reason is fallible and therefore cannot be used as a benchmark to determine the reality of established truths. Philosophical skeptics doubt whether they can ever obtain enough convincing evidence (or justification) to support popular assumptions.

Philo maintains that human reason alone is insufficient to determine the validity of religious claims:

Let us become thoroughly sensible of the weakness, blindness, and narrow limits of human reason: Let us duly consider its uncertainty and endless contrarieties, even in subjects of common life and practice...When these topics are displayed in their full light...who can retain such confidence in this frail faculty of reason as to pay any regard to its determinations in points so sublime, so abstruse, so remote from common life and experience?

Philo claims that it's fine to rely on "common sense and experience" when it comes to mundane matters such as "trade, or morals, or politics, or criticism." After all, we're dealing with tangible concepts here. However, he argues that human reason can't accurately determine how "the creation and formation of the universe" really came about. Philo questions how we can prove with any degree of certainty "the existence and properties of spirits; the powers and operations of one universal Spirit existing without beginning and without end; omnipotent, omniscient, immutable, infinite, and incomprehensible..."

It should be noted that Philo isn't proposing absolute skepticism here (absolute skepticism is the belief that truth cannot be determined). However, he is arguing that, in matters of theology, we humans have no concrete way of determining the validity of absolute truths.

For his part, Cleanthes argues that faith is essential when it comes to matters of theology. He aims to point out the hypocrisy in Philo's position of philosophical skepticism, especially in this area. Cleanthes puts forth the argument that Philo accepts scientific truths that are speculative in nature (for the time, at least). So, he questions why Philo cannot transfer this openness to matters of theology. Here are his words:

In reality, would not a man be ridiculous, who pretended to reject NEWTON's explication of the wonderful phenomenon of the rainbow, because that explication gives a minute anatomy of the rays of light; a subject, forsooth, too refined for human comprehension? And what would you say to one, who, having nothing particular to object to the arguments of COPERNICUS and GALILEO for the motion of the earth, should withhold his assent, on that general principle, that these subjects were too magnificent and remote to be explained by the narrow and fallacious reason of mankind?

So, in questioning Philo's attempt to erect religious faith on philosophical skepticism, Cleanthes aims to expose the dangers of a "brutish and ignorant skepticism" that will "reject every principle which requires elaborate reasoning to prove and establish it." Cleanthes contends that such skepticism is "fatal to knowledge." He invites Philo to reconsider his views.