“[K]nowledge that is not Infallible is not certain knowledge.”

The Virtue of Selfishness (1964)

Source: 1960s - 1970s, The Design of Inquiring Systems (1971), p. 11

Matthew Stewart, The Courtier and the Heretic (2006)
Context: Like Socrates, Spinoza avers that blessedness comes only from a certain kind of knowledge—specifically, the "knowledge of the union that the mind has with the whole of Nature."
... the life of contemplation is also a life within a certain type of community—specifically, a fellowship of the mind. Like Socrates with his circle of debating partners, or Epicurus in his garden with his intellectual companions, Spinoza imagines a philosophical future... upon achieving blessedness for himself, he announces in his first treatise, his first step is "to form a society... so that as many as possible may attain it as easily and as surely as possible." For, "the highest good," he claims, is to achieve salvation together with other individuals "if possible."

As quoted in Wilhelm von Humboldt (1970), by P. Berglar, p. 87, and "Profiles of Educators: Wilhelm von Humboldt (1767–1835)" by Karl-Heinz Günther, in Prospects, Vol. 18, Issue 1 (March 1988)
Context: There are undeniably certain kinds of knowledge that must be of a general nature and, more importantly, a certain cultivation of the mind and character that nobody can afford to be without. People obviously cannot be good craftworkers, merchants, soldiers or businessmen unless, regardless of their occupation, they are good, upstanding and – according to their condition – well-informed human beings and citizens. If this basis is laid through schooling, vocational skills are easily acquired later on, and a person is always free to move from one occupation to another, as so often happens in life.

Source: 1910s, The Problems of Philosophy (1912)

The Last Trip To Woody Creek, p. 376
The Joke's Over (2006)

Environmentalism as a Religion (2003)
Context: The second reason to abandon environmental religion is more pressing. Religions think they know it all, but the unhappy truth of the environment is that we are dealing with incredibly complex, evolving systems, and we usually are not certain how best to proceed. Those who are certain are demonstrating their personality type, or their belief system, not the state of their knowledge.

Source: The Strength To Dream (1961), p. 197
Context: No artist can develop without increasing his self-knowledge; but self-knowledge supposes a certain preoccupation with the meaning of human life and the destiny of man. A definite set of beliefs — Methodist Christianity, for example — may only be a hindrance to development; but it is not more so than Beckett's refusal to think at all. Shaw says somewhere that all intelligent men must be preoccupied with either religion, politics, or sex. (He seems to attribute T. E. Lawrence's tragedy to his refusal to come to grips with any of them.) It is hard to see how an artist could hope to achieve any degree of self-knowledge without being deeply concerned with at least one of the three.