“The plague of man is boasting of his knowledge.”

Attributed

The Coming of Love and Other Poems (1897)
Source: "Prophetic Pictures at Venice VII: New Year's Morning, 1867", p. 207.

The Tragic Sense of Life (1913), III : The Hunger of Immortality

1900s, The Strenuous Life: Essays and Addresses (1900), National Duties
Context: In private life there are few beings more obnoxious than the man who is always loudly boasting; and if the boaster is not prepared to back up his words his position becomes absolutely contemptible. So it is with the nation. It is both foolish and undignified to indulge in undue self-glorification, and, above all, in loose-tongued denunciation of other peoples. Whenever on any point we come in contact with a foreign power, I hope that we shall always strive to speak courteously and respectfully of that foreign power.

Source: Drenai series, Legend, Pt 1: Against the Horde, Ch. 7

Knowing Yourself: The True in the False (1996)
Context: You think: you become that thought. And consciousness, or the state of pure awareness, is lost. The highest knowledge man can possess is that which is true in his own experience. If his experience is limited, so is his knowledge and he behaves accordingly.

Source: Carlos Castaneda (1971) Separate Reality: Conversations With Don Juan. p. 85; As cited in: Eugene Dupuis (2001) Time Shift: Managing Time to Create a Life You Love. Ch. 5: Self Management

Book II, Ch. 1, sec. 19
An Essay Concerning Human Understanding (1689)