Besides being a philosopher I am also a gardener and I grow apples. It’s autumn, so recently I have harvested my apples, too many apples to eat all within a few weeks. Therefore I must store them. I regularly check them for sometimes an apple has started to rot. I remove it immediately, for it can infect others. Usually, only a few apples will become rotten, so for months I can enjoy eating apples from my own garden. So why should I throw away all once I have discovered a rotten apple, because it might have infected the others already, or because they might rot later?
It's what René Descartes did. When Descartes wondered if everything he thought he knew – through his senses, dreams or an evil impostor – could be an illusion, he thought the best he could do was completely empty his mind and discard all knowledge. For this was the only way that he could be sure that there was no false and fake knowledge left in his mind that could lead him into error and make him believe that what was false or fake would be true. Then starting to think from nothing, Descartes concluded that only his existence was certain, which led him to the famous saying “Cogito ergo sum”, I think therefore I am.
Pierre Gassendi (1592-1655) was not convinced that Descartes’ method was right: “[Why didn’t you] make a simple and brief statement to the effect that you were regarding your previous knowledge as uncertain so that you could later single out what you found to be true. Why instead did you consider everything as false, which seems more like adopting a new prejudice than relinquishing an old one? This strategy made it necessary for you to convince yourself by imagining a deceiving God or some evil demon who tricks us, whereas it would surely have been sufficient to cite the darkness of the human mind or the weakness of our nature.” In a letter Descartes replied that to find the right foundation of knowledge we must “demolish everything completely and start again right from the foundations”. In explaining why he came up with an analogy:
“Suppose [a person] had a basket full of apples and, being worried that some of the apples were rotten, wanted to take out the rotten ones to prevent the rot spreading. How would he proceed? Would he not begin by tipping the whole lot out of the basket? And would not the next step be to cast his eye over each apple in turn, and pick up and put back in the basket only those he saw to be sound, leaving the others? In just the same way, those who have never philosophized correctly have various opinions in their minds which they have begun to store up since childhood, and which they therefore have reason to believe may in many cases be false. They then attempt to separate the false beliefs from the others, so as to prevent their contaminating the rest and making the whole lot uncertain. Now the best way they can accomplish this is to reject all their beliefs together in one go, as if they were all uncertain and false. They can then go over each belief in turn and re-adopt only those which they recognize to be true and indubitable.”
Descartes’ answer to Gassendi has become known as the “Bad Apple Theory”. In short: “Because one bad apple can spoil the whole bunch, the only sure means to a rot-free basket is to discard the whole lot”, as the Stanford Encyclopedia of Philosophy explains.
Although Descartes means here that we must not trust the ancient authorities who tell us what true knowledge is and that we must not believe that our senses are reliable, as an apple grower I have my doubts. When I threw away all my apples, once I had discovered a rotten one to be sure that no possible rotten apples would be left, I would have nothing to eat and I would starve to death. I need to eat the old apples, even if they are possibly rotten, to survive so that I can find a manner to prevent that apples will become rotten in the future again. I need the possibly rotten apples in my struggle against rottenness. So, although basically Descartes is right, we simply cannot discard all knowledge and start anew from nothing. If there is nothing, there is nothing to start from. An empty mind cannot think. We simply need thinking stuff, even if it is old knowledge that is partly false or fake, to be able to get better knowledge. We stand on the shoulders of others as Isaac Newton famously said. How should we imagine that?
Someone who tackled the problem was Karl R. Popper (1902-1994), who designed a scheme for developing better knowledge from old possible false or fake knowledge:
What Descartes actually did by rejecting all beliefs together in one go was not only destroying all apples but all apple trees as well, for isn’t it so that all existing apple trees produce apples that are vulnerable to rottenness? So to be sure to have apples that are not possibly rotten, away with the trees as well! The only way to get rot-free apples is from the seed from old apple varieties and to cross-fertilize them and hoping to get rot-free apples in this way. It’s a matter of trial and error. But you can do this only if you maintain your old orchard yet for a little while.
Sources
- “Descartes’ Epistemology”, https://plato.stanford.edu/archives/fall2007/entries/descartes-epistemology/, section 2.2 for the references to Gassendi and Descartes and the quotes there.
- For Poppers theory, see my blog “When science fails”.









