Russell opened The Problems of Philosophy with a question: ‘Is there any knowledge in the world which is so certain that no reasonable man can doubt it?’ Russell was forty when the Problems were published (in 1912).
The nonagenarian Russell wrote in the ‘Postscript’ to his Autobiography that up to the age of thirty-eight he gave most of his energies to finding out whether anything could be known: ‘I wanted certainty in the kind of way in which people want religious faith. I thought that certainty is more likely to be found in mathematics than elsewhere. But I discovered that many mathematical demonstrations, which my teachers expected me to accept, were full of fallacies, and that, if certainty was indeed discoverable in mathematics, it would be in a new kind of mathematics, with more solid foundations than those that had hitherto been thought secure. But as the work proceeded, I was continually reminded of the fable about the elephant and the tortoise. Having constructed an elephant upon which the mathematical world could rest, I found the elephant tottering, and proceeded to construct a tortoise to keep the elephant from falling. But the tortoise was no more secure than the elephant.’
The septuagenarian Russell says in the ‘Introduction’ to his History of Western Philosophy (first published in 1946): ‘To teach how to live without certainty, and yet without being paralysed by hesitation, is perhaps the chief thing that philosophy, in our age, can still do for those who study it.’