"Some one had blundered."

This line is uttered by Mr. Ramsay and reiterated multiple times in the first few sections of To The Lighthouse, so it is worth giving extra attention. The two most recent quotations in the OED for the verb "blunder" correspond to the definition "To bring or cause to fall into a state by clumsy or inept behaviour." This is the common usage we encounter, with a blunder being a mistake resulting from carelessness or some other oversight.

However, the older definitions of "To confound, distract (in understanding)," and " To confound (in one's mind) stupidly," offer a much different meaning of the sentence. The implication is of being mentally overthrown, destroyed. In this case, there is no clumsy behavior. The blunderer is simply overpowered in his or her understanding.

Reading this statement coming from a character who is a philosopher may shed some light on the episode that takes place in the beginning of the novel. "The Ramsays were not rich, and it was a wonder how they managed to contrive it all." Mr. Ramsay raised his eight children on philosophy alone. If he were to suffer a defeat in his reasoning or understanding, it would be catastrophic to his livelihood. If the blunder was perpetrated by some other philosopher whom Ramsay based his own work off, it could create a philosophical crisis for him -- what he based his life on could be disassembled at a moment's notice.