Though Erdős used this remark, it is said to have originated with his friend Stanisław Ulam, as reported in The Man Who Loved Only Numbers : The Story of Paul Erdős and the Search for Mathematical Truth (1998) by Paul Hoffman
Variants:
The first sign of senility is when a man forgets his theorems. The second sign is when he forgets to zip up. The third sign is when he forgets to zip down.
As quoted in Wonders of Numbers : Adventures in Mathematics, Mind, and Meaning (2002) by Clifford A. Pickover, p. 64
There are three signs of senility. The first sign is that a man forgets his theorems. The second sign is that he forgets to zip up. The third sign is that he forgets to zip down.
Misattributed
“Doubt is not always a sign that a man is wrong; it may be a sign that he is thinking.”
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Oswald Chambers 43
British missionary 1874–1917Related quotes
Attributed in Paul Hoffman, The Man Who Loved Only Numbers: The Story of Paul Erdős and the Search for Mathematical Truth (1998)
This has also been attributed, with variants, to Paul Erdős, who repeated the remark.
Oeuvres Complètes, vol. 3. L’Avenir de la Science (1890).
The Meaning of Immortality in Human Experience (1957), p. 5.
Context: Man is the only animal that contemplates death, and also the only animal that shows any sign of doubt of its finality. This does not mean that he doubts it as a future fact. He accepts his own death, with that of others, as inevitable; plans for it; provides for the time when he shall be out of the picture. Yet, not less today than formerly, he confronts this fact with a certain incredulity regarding the scope of its destruction.
On George W. Bush
Interview with David Brancaccio (2003)
418
1940s–present, Minority Report : H.L. Mencken's Notebooks (1956)
“The surest sign that a man has a genuine taste of his own is that he is uncertain of it.”
"Reading", p. 6
The Dyer's Hand, and Other Essays (1962)
Source: "Foundations of the Theory of Signs," 1938, p. 3