It is astonishing what force, purity, and wisdom it requires for a human being to keep clear of falsehoods.

Fuller’s sentence treats truthfulness not as a passive default but as an active moral and intellectual achievement. “Force” suggests the social pressure and self-interest that tug people toward convenient distortions; “purity” points to the need for motives uncorrupted by vanity, fear, or faction; and “wisdom” implies discernment—knowing how easily one can mistake prejudice, rumor, or self-deception for fact. The remark also hints at a Transcendentalist concern with integrity of mind: to “keep clear” of falsehoods requires continual vigilance, because untruth can arise not only from deliberate lying but from muddled thinking and compromised character.