His half-sister, Gully Wells, who is 12 years older and grew up in the same household, gave a more positive view. “He moved into our house when I was about five,” she told me. “And there he was, this gentle, sweet man, who was always interested in me, even when I was young.” Later on, she would share his interest in literature and history, and he’d sometimes help her with her schoolwork. “My mother would have said he was a rather emotionally remote man to be married to. He was probably an easier step-father than he was a husband.” She referred to his multiple affairs, which, like Russell, he had throughout his life.

Russell’s daughter often felt like a disappointment to her father, who expected great things from his children; Ayer’s son, who did not share his father’s interest in philosophy, described to me how he was able to impress his father once, when asking him if a toy trumpet made a real sound.

Among the more encouraging examples of philosopher fathers was the Stoic Epictetus, who, at an old age, adopted a friend’s child and angrily defied Epicurus’s assertion that wise men should not bring up children. Then there was the American philosopher John Dewey, who adopted two children very late in life and was dedicated to his family, spending much of his money on travel with them, often to Europe, and on buying books for the children. “I can think of no major philosopher other than Dewey who had a lifelong intimate relation with his children, and especially one that so clearly influenced his writings all his career,” his biographer, Jay Martin, wrote to me in an email. Dewey wrote extensively on the education of children, but, as with most other philosophers, he never addressed the question of fatherhood head-on.

Contemporary philosophers do not seem much more interested in the topic. With the exception of the 2010 collection “Fatherhood: Philosophy for Everyone,” very little has been said about fatherhood. Whatever the reason, it isn’t childlessness. I sent out a request to 12 leading philosophy institutions in the United States. Based on the responses from seven of them, encompassing a little more than 100 faculty members, I found that more than 75 percent had children, the same for women as for men. All seven philosophy department heads said that the degree of childlessness was not greater in their departments than in others.

So why had philosophers been so reluctant to write about fatherhood, and so ineptly, if not sexist, in the few cases they had done so? I decided to ask a woman, Kelly Oliver, a philosophy professor at Vanderbilt University:

“The history of philosophy is the history of patriarchal power over both the mind and body,” she wrote to me in an email. “Fatherhood has been the unquestioned foundation of familial and civil law, and as such, has not been investigated until relatively recently, when patriarchy in philosophy and beyond has been challenged by philosophers influenced by feminism and psychoanalysis.”

In their work, Levinas, Ricoeur and Derrida challenged the patriarchal notion of the father, as a symbol for authority, but they all remained silent on their own experiences of fatherhood. But there are, of course, contemporary philosophers who have given the subject some thought.