Dualism & Leibniz's Law
Entering into the phenomena of the mind-body connection debate
Dualism
Substance dualism is defined by the argument that for any domain, there exists two kinds of foundational elements: mental substance and physical substance. This view, most strongly supported by French Philosopher Descartes, is then used to explain that the mind (mental substance) can exist outside of the body, but yet the body (physical substance) cannot think for itself. Thus, substance dualism argues, the mind can live on past the deterioration of the body. This is sometimes referred to as the idea of a ‘soul’.
On the contrary, property dualism holds the stance that the mind and the body are inextricably linked. Everything, then, can be said to be made up of physical substance. This physical substance can be further categorized into physical and mental properties, however they are just pieces of a whole. Consequently, it follows that when the physical body is destroyed, both the physical and mental properties– mass, size, beliefs, feelings, pain, guilt– that constitute the physical substance will perish together.
Leibniz’s Law
Leibniz’s law is a principle concerning the ontological existence of two separate entities. In simple terms, it states if two entities have the same properties, then they are the same thing; similarly, if two entities have even one differing property, they must be different things. In reference to Descartes’ argument for Dualism, Leibniz’s law can be cited in the following analogous argument: Physical states have a physical mass, but mental states have no mass. Therefore physical and mental states must be different things. This argument is valid as it follows the law and makes logical sense, however this argument may not necessarily be sound. I propose that Leibniz’s Law may also be used in the argument against Dualism. Inspired by Braddon-Mitchell and Jackson’s chapter “Flight From Dualism”, I entertain the thought that mental properties could have mass, we just don’t have the tools to measure them yet. The authors refer to the fact that not too long ago, the idea that something like light could have a mass was an unthinkable concept. Now though, with the help of “ultramodern technology'', we are able to measure the mass of light with such ease that the concept can be taught to children in grade school. To quote BM&J, discovering a mode of measurement for mental states “wouldn’t be the most unheard of thing in the entire world”. The mere possibility that this could be the case, fractures Descartes’ argument for Dualism that mental and physical properties are not the same thing.
Take the argument:
“I believe that Mohammad Ali is a great boxer. I do not believe that Cassius Clay is a great boxer. Therefore, Mohammad Ali is not Cassius Clay.”
How can we decide whether or not this argument is valid?
This argument attempts to follow Leibniz’s Law in such that if two entities, in this case Mohammad Ali and Cassius Clay, have different properties, they must be two different entities. However, this cannot be valid due to its neglect of specification between differing properties. The only difference in the case given regards the speaker’s beliefs. Logical arguments must stand upon propositions which are the foundations of premises. Propositions are statements that are assigned a truth-value; that is, the statement is either true or the statement is false. There is no gray area. Beliefs, though, are a gray area. Beliefs are boundless, their very nature is that they are allowed to hold true even when evidence points to the contrary: people will continue to believe god created man through evidence of civilizations from thousands of years before the story of god had first been told. Version 1 of Descartes’ argument from doubt falls to a similar plight. Descartes begins his argument, encouraging people to doubt everything in order to find the beliefs that are most firm. So, he says, everything about the physical world is subject to doubt. The only thing you cannot doubt is your mind. Therefore, he argues, the physical aspects of your body and brain are not the same as the mental aspects of the mind. As was said before, believing something exists about you or doubting that it doesn’t, without actually specifying the exact differing properties of concern is not grounds enough for a valid argument. Thus, this is an incorrect application of Leibniz’s Law.
Related Information
Braddon-Mitchell and Jackson: “The Flight from Dualism” from Philosophy of Mind and Cognition
René Descartes 1639: Meditations on First Philosophy
Leibniz’s Law: The Identity of Indiscernibles