I don’t think so I am not?

I start with a joke:

René Descartes is in a tavern. He is drunk. The bartender cautions: “Monsieur, I think that you have had enough.” Descartes slurs back “I think not” and vanishes.

I hope that you understood and that you laughed, but now I become serious, for the reasoning implied is fallacious: Descartes’s famous statement “I think so I exist” may be true, but it does not imply that I don’t exist if I don’t think. And it is not so that if I exist this implies that I think. For would it be so then that I don’t exist when I am sleeping? And how about a tree? But okay, as for the latter you can object that a tree hasn’t an “I” (which is something to discuss about in another blog).

In philosophy we call this fallacy post hoc ergo propter hoc, literally “after this so because of this”. In this case the fallacy is the reasoning that I exist because I think. However, thinking is only an aspect of human existence, but the cause of my existence must be found elsewhere.

I don’t know whether Descartes would have laughed about the joke, but actually he worried about the question whether he would exist, if he didn’t think. Although the “I think so I exist” doesn’t imply that I exist because I think, nevertheless it is quite well possible that I don’t exist if I don’t think. Descartes worded his worry this way in the “Second Meditation” of his Meditations on First Philosophy:

“I am; I exist – this is certain. But for how long? For as long as I am thinking; for perhaps it could also come to pass that if I were to cease all thinking I would then utterly cease to exist. At this time I admit nothing that is not necessarily true.”

On purpose I have quoted Descartes from Sorenson’s A Cabinet of Philosophical Curiosities where he gives an explanation of this passage. For immediately after it Sorenson continues:

“Compare Descartes’s principle connecting thinking and human existence with another Cartesian principle connecting being extended in three dimensions with physical existence: Necessarily if a body exists, it is extended in space. If a physical thing ceases to be extended, then it ceases to exist. Similarly, if a mind exists, it thinks. And if the mind ceases to think, then it ceases to exist.”

So, after all must we conclude that if we don’t think then we don’t exist, albeit on other grounds than the false post hoc ergo propter hoc reasoning? Not so, for here we have the fallacy of false analogy. Above I raised the question whether one still exists if one sleeps (and so doesn’t think). The answer is, of course, “yes”. Why? Because thinking is not a property that a mind simply has and manifests itself but it is a disposition: a property an object has even if this property is not active at a certain moment. This makes thinking different from the property “being extended”, which points to the way an object appears (in this case by having the three dimensions width, height and depth). If the disposition “thinking” isn’t active, it still doesn’t mean that it doesn’t exist as a disposition. A glass remains brittle, even if it will never fall from the table on the floor and break, although it is always possible that this will happen. But if we can’t pour water in it, because it is not extended, it is not a glass. It is similarly with thinking. Even if a mind doesn’t think, the property “thinking” is still there and the mind can start to think when it needs to, for example when one wakes up in the morning. When a man is absent-minded, it doesn’t imply that s/he hasn’t a brain.

Sources

- Roy Sorensen, A Cabinet of Philosophical Curiosities. A Collection of Puzzles, Oddities, Riddles and Dilemmas. London: Profile Books, 2017. The quotes are from pp. 42-43.

- There are several versions of the joke on the Internet. You can find it also in Sorensen’s book on p. 42.