Reason means truth and those who are not governed by it take the chance that someday the sunken fact will rip the bottom out of their boat.

Holmes equates “reason” with an uncompromising commitment to reality: to be rational is to be governed by what is true, not by wish, habit, or ideology. The metaphor of a “sunken fact” suggests that inconvenient truths can be ignored for a time—lying beneath the surface like a hidden obstruction—but they retain the power to cause sudden catastrophe. In legal and civic life (Holmes’s central arena), the warning reads as a critique of decision-making driven by sentiment, dogma, or expediency: policies and judgments that refuse to reckon with facts may appear to float, yet they risk eventual failure when reality asserts itself. The line captures Holmes’s pragmatic, unsentimental cast of mind and his suspicion of comforting illusions.