“To know that we know what we know, and that we do not know what we do not know, that is true knowledge.”

Source: Walden

Source: Value-free science?: Purity and power in modern knowledge, 1991, p. 13

Source: Tertium Organum (1912; 1922), Ch. I
Context: The most difficult thing is to know what we do know, and what we do not know.
Therefore, desiring to know anything, we shall before all else determine WHAT we accept as given, and WHAT as demanding definition and proof; that is, determine WHAT we know already, and WHAT we wish to know.
In relation to the knowledge of the world and of ourselves, the conditions would be ideal could we venture to accept nothing as given, and count all as demanding definition and proof. In other words, it would be best to assume that we know nothing, and make this our point of departure.
But unfortunately such conditions are impossible to create. Knowledge must start from some foundation, something must be recognized as known; otherwise we shall be obliged always to define one unknown by means of another.

How Children Fail (1964).

"Ce que nous connaissons est peu de chose, ce que nous ignorons est immense."
Allegedly his last words, reported in Joseph Fourier's "Éloge historique de M. le Marquis de Laplace" (1829) with the comment, "This was at least the meaning of his last words, which were articulated with difficulty." Quoted in Augustus De Morgan's Budget of Paradoxes (1866).

Part of this quote may actually be by Ralph Washington Sockman.
The World's Religions (1991)
Source: Beyond the Post-Modern Mind: The Place of Meaning in a Global Civilization
Context: In mysteries what we know, and our realization of what we do not know, proceed together; the larger the island of knowledge, the longer the shoreline of wonder. It is like the quantum world, where the more we understand its formalism, the stranger that world becomes.

Rules 2 Rule

Source: Teaching Critical Thinking: Practical Wisdom