Philosophy Should Not Be Just an Academic Discipline: A Dialogue with Hilary Putnam

János Boros: Let us begin with a nonacademic question. What is your relationship to the movies The Matrix and The Matrix II?

Hilary Putnam: In the book Reason, Truth, and History, which I published about twenty years ago, the first chapter was titled "Brains in a Vat." It had basically the same scenario as The Matrix does, the scenario in which all human beings, all sentient beings, exist in a vat. Their sensations are totally controlled by a computer. I was not aware that the producers of The Matrix had read my work. It seemed possible that they came on this idea by themselves; but, in fact, before The Matrix II was released, I was approached and asked whether I was willing for my chapter to be listed in references on the Matrix II website. I gave my permission.

Boros: Which leads me to a second question. You quite often work with thought experiments of the kind that are commonly used in natural sciences but not in philosophy. What do you think is the role of thought experiments in philosophical argument?

Putnam: I think that in natural sciences, when they occur there, thought experiments typically are used for conceptual clarification. For example, Newton used a famous thought experiment to argue that there is a difficulty with the philosophical view, defended by Leibniz, that motion and rest are simply relative—the difficulty is that if all motion is simply relative, then acceleration must be relative too. However, Newton pointed out, there are physically observable differences between an accelerating system and a system that does not accelerate. This objection to the relativity of motion could not be answered within the scope of seventeenth- century physics. It took a very different physics, the theory of general relativity, to answer Newton's objection to a relativistic account of motion. One could make the point without the dramatic thought experiment Newton used. Newton imagined a world that contains nothing but a bucket half filled with water and asked whether there isn't a difference between the bucket being at rest and the bucket spinning around. There is a difference between the two situations—if the bucket is spinning, the water climbs the sides of the bucket! The thought experiment makes a dramatic point. One can make the same point without it, but doing so would take many pages. I think that in philosophy the function of thought experiments is similar—they are dispensable, but they enable us to see conceptual points clearly and quickly.

Boros: There is an opinion in circulation that some of your thought experiments have practically zero mathematical probability or that they are physically impossible, and so they are unconvincing even if they are logically correct. Do you think that, in philosophical argumentation and in philosophical thought experiments, logical consistency is enough, or do we need physical consistency as well? By this, I mean that your thought experiments—"brains in a vat" and "twin earths (with XYZ 'water')"—are physically inconsistent; they simply contradict known physical laws. Brains have three billion neurons with combinatorial connections—in other words, with a "combinatorial explosion." Hence, they are so complex that a "fine-tuned" control from outside is not possible. Among other problems, every computation of every single brain state would take a (combinatorially exploded) long time. Life, as we know, is built up on H₂O, water; and a life built on an XYZ other than water would be completely different from ours. If Descartes's cogito is coherent, it must be coherent in a logical sense and in a physical sense, where "physical" means the "inside physics" of the cogito, "the physics in my mind." Otherwise the cogito would contradict its own "internal" world and every argument about it would be invalid.

Putnam: In the case of "brains in a vat," the argument that it is physically impossible is irrelevant, because this scenario is just a dramatization of the Cartesian argument. Sure, you can say to the Cartesian skeptic: what you imagine is physically impossible...