Justifying the physical world
Descartes’ argument for the reality of the physical world is this
We have experiences of various things which we take to be physical objects.
Either these experiences refer to real objects, or some powerful being is causing me to falsely believe them.
If some powerful being is causing me to believe these things, then that being is a deceiver and is not a good being.
However, I have proved that a perfect being exists and is all-powerful.
Such a perfect being would not deceive me, and would not permit a less powerful being to deceive me in such a massive way.
Therefore, the experiences are not deceptions, and physical objects really do exist.
Some questions about this
If God is omniscient, omnipotent, and perfectly good, then on the face of things God should not allow physical evil (suffering) as well as intellectual evil (error or false beliefs).
But there is physical evil and intellectual evil. Therefore either an omnipotent, omniscient and perfectly good being doesn’t exist, or He (or She) has a good reason for permitting both physical and intellectual evil.
Therefore, either God does not exist, or God’s existence is compatible with the existence of intellectual evil.
But how massive a deception is possible if there’s a God?
Descartes perhaps thinks that God’s existence is incompatible with really massive and systematic intellectual evil, such as we would have if we were deceived about the existence of the physical world.
But there are some people down on 24th and Van Buren (the local mental hospital in Phoenix) who think they are poached eggs, or think that they are Napoleon, or think they’re Jesus Christ.
Our belief in the physical world
Descartes does point out that physical objects are not always as they appear to the senses.
However our minds have the ability to overcome the defects of sensory knowledge. For example, even though a straight stick in water appears to the eye to be bent, we nevertheless can figure out that it really is straight.
Descartes on physical things
Physical things “possess all the properties which I clearly and distinctly understand, that is, all those which, viewed in general terms, are comprised within the subject-matter of pure mathematics.”
Descartes and Galileo
A. Descartes is supporting Galileo’s view that the real nature of the physical world is mathematical. The objective properties of an object are the properties that can be given a mathematical description, such as size, shape and motion. Colors, tastes, sounds and all the other qualities of our experience are in the mind but not in objects themselves. The world as it is in itself is colorless, but it has the tendency to produce in our minds a sense of it being this color or that color.
The mind-body relation
Descartes accepts the medieval (and Aristotelian) conception of substance.
At first he defines substances as something that exists in such a way as to depend on nothing for its existence. This definition would mean that only God is a substance.
But other substances are call that by analogy.
There are two types of substances: mental substances and physical substances. This implies that the mind and the body are two completely different entities.
Why they are so different
The body can be doubted but the mind cannot be doubted.
Minds have complete different attributes than physical things
1. Bodies take up space and are move by physical forces.
2. Minds do not take up space and are a kind of nonphysical or spiritual reality.
3. Minds are not made up of parts and cannot be divided.
How are these types of reality linked together
The body is a machine made out of flesh and bone.
All animals but humans are just machines.
Thus you can’t really be cruel to animals, since they don’t really have any feelings. (This puts Descartes in the PETA hall of shame).
C. Your mind is the real you. You don’t have a mind you are a mind. You have a body.
Cartesian Dualism
Cartesian dualism is a metaphysical dualism. It is also a mind-body dualism. There are two irreducible types of realities.
By means of this kind of dualism, Descartes hoped to effect a compromise between religion and science. One part of reality, the physical part, can be analyzed by physical methods. It is an entirely clocklike mechanism, governed by the laws of nature, with no purpose whatsoever. The other part of reality, the mind, has freedom to think and will as we wish, and is not under the control of physical law.
In the physical realm, science is the authority and gives us the truth, but it tells us nothing about the eternal destiny of our souls, only about our bodies. In the spiritual realm, religion remains authoritative.
Relating the two realms
There remains, however the issue of how these two realms were related. If I commit a murder, I may begin with a homicidal thought, which presumably exists in my soul, but then that homicidal thought has to result in my brain sending a message to my trigger finger to go ahead and fire and leave my victim in a pool of blood. In order for this to be even possible, there has to be some kind of causation between the mental realm and the physical realm. Therefore, if Cartesian dualism is true, it is not quite the case the physics fully explains the physical. It fully explains the physical unless the mental interferes with it.
How do they interact?
Descartes further is faced with the problem of how the mental realm and the physical realm can interact, since they are so different. As Lawhead’s text says “The mind has no gears of muscles or chemicals by which to move other things or to be moved.”
Descartes’ solution makes things worse. He suggests that the pineal gland at the base of the brain, which at the time no one knew what it did, and that the pineal gland was affected by “vital spirits.” But this really doesn’t solve the problem.
An overrated objection?
William Hasker, who is a contemporary dualist (but a dualist of a different kind from Descartes) argues that this objection is overrated. He writes:
Hasker’s response
The hoariest objection specifically to Cartesian dualism (but one still frequently taken as decisive) is that, because of the great disparity between mental and physical substances, causal interaction between them is unintelligible and impossible. This argument may well hold the record for overrated objections to major philosophical positions. What is true about it is that we lack any intuitive understanding of the causal relationship between Cartesian souls and bodies. And there is no doubt that, other things being equal, a mind-body theory that allowed such understanding would seem preferable to one that did not. The reason this is not decisive is that, as Hume pointed out, all causal relationships involving physical objects involving physical objects are at bottom conceptually opaque. We find the kinetic theory of gases, with its ping-pong-ball molecules bouncing off each other, fairly readily understandable.
Hasker continues
This, however, is only because we have learned from experience about the behavior of actual ping-pong balls, and our expectations in such cases have become so habitual that they seem natural to us; we have no ultimate insight into the causal relations except to say, “That’s the way things are.” But equally and emphatically, “the way things are” includes the fact that our thoughts, feelings, and intentions are affected by what happens to our bodies, and vice versa, and to deny these palpable facts for the sake of a philosophical theory seems a strange aberration.
Keith Parsons disagrees (in a two-journal exchange with me)
It will simply not do for Hart (or Reppert) to take refuge in familiar Humean conundrums about causality. Much of the progress of science has been progress in understanding how things interact: Plate tectonics tells us how crustal plates interact to produce earthquakes, volcanoes, mountains, and other geological phenomena. Likewise, ecology helps us to understand the enormously complex interactions of organisms with their physical environments and each other. Molecular biology explains the interactions of complex molecules, e.g. enzymes and their substrates. Even at the rock-bottom level of quarks and gluons we have well-confirmed, mathematically precise theories that often make (as in Quantum Electrodynamics) astonishingly accurate predictions. These theories tell us how fundamental particles interact.
Another Parsons comment
Descartes took this objection seriously, and he should have. Surely dualists owe the rest of us some sort of account. After all they posit and entity that has no physical properties (and consequently is undetectable by any empirical means), but which is not an abstract entity since it somehow interacts with physical things—in a way that violates conservation laws, by the way. Souls could not have been produced by physical means, and their putative existence raises a host of unanswerable questions. (For example, at what point at the evolution of hominids did our ancestors acquire souls? Homo habilis or Homo erectus, maybe As Lycan notes above, Descartes took this objection ).
Alternatives to Interactionism (Descartes didn’t buy these)
Geulincx said that mental events and physical evens are independent processes that only seem to influence one another. But actually God arranges two parallel series of mental and physical events. This is parallelism
Malebranche said that each type of event is an occasion on which God produces correlated events in the other realm. This is occasionalism.
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