On the Nature of Things

Motion of Atoms Weight of atoms causes them to naturally move downward (II, 190). All atoms have equal speed in void (II, 239).

Galileo ( )

Motion of Atoms So why isn’t reality one giant hail storm of atoms?

Lucretius’ Universe

Motion of Atoms Democritus held that atoms move in no natural direction, but move in all different directions—so collision is common.

Democritus’ Universe

Motion of Atoms Democritus held that atoms move according to strict laws. Deterministic system: A system of objects is deterministic if, given the position, speed and direction of movement of each object in the system at a given time, there is one and only one state (set of positions, speeds and directions of the objects) the system could be in at a given later time.

Motion of Atoms A deterministic system is one that is completely governed by strict physical laws. Isaac Newton’s laws of motion. The past states of the system completely determine the future states of the system. If a system is deterministic, and we know its state at a given time, it is possible, in principle, to predict its state at any later time.

Pierre LaPlace ( ) French philosopher & mathematician:

“We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at any given moment knew all of the forces that animate nature and the mutual positions of the beings that compose it, if this intellect were vast enough to submit the data to analysis, could condense into a single formula the movement of the greatest bodies of the universe and that of the lightest atom; for such an intellect nothing could be uncertain and the future just like the past would be present before its eyes.”

Einstein: “God does not play dice with the universe.”

Motion of Atoms For Democritus, LaPlace and Einstein, our universe is a deterministic system. But Lucretius believed that all atoms naturally travel downward, at the same speed. So how can there be any interaction or collision between atoms?

Motion of Atoms Atoms must occasionally swerve slightly! (II, 220) Swerve is rare, slight, and unpredictable.

Lucretius’ Universe

Motion of Atoms Swerves cause collisions and interactions. Are swerves detected empirically? No (II, 249). Lucretius hypothesizes swerves to explain why there is interaction between atoms. What contemporary view in physics does the swerve remind you of? Quantum Mechanics

Werner Heisenberg German physicist ( )

Motion of Atoms Heisenberg Uncertainty Principle: At the most basic atomic level, there is a degree of uncertainty in the laws of physics. Indeterministic system: A system is indeterministic if, given its state at one time, there is not only one state it could be in at a given later time. In an indeterministic system it is not possible, even in principle, to predict its precise state at a future time.

Motion of Atoms At the most basic level, the laws of physics are probabilistic, not deterministic. This does NOT mean that there are no laws of physics, or that the universe is totally random! Swerves and quantum indeterminacy happen at the atomic level—not at the observable level. What we call laws are really statistical approximations.

Motion of Atoms For Lucretius and Heisenberg, our universe is an indeterministic system.

Atomic Theory of the Soul Soul is made of atoms, just like everything else (III, 161). Soul atoms are very small, light-weight, smooth, and fast-moving (III, 203). Soul atoms diffused throughout body, centering on the heart!

Atomic Theory of the Soul Argument that soul is physical (III, 162-8): Soul causes physical changes in world (e.g., I decide to move my arm, so it moves). All change is caused by the physical impact of one body on another (I, 304).  Soul is physical.

Atomic Theory of the Soul Suppose soul is made of physical atoms. Suppose we agree with Democritus that the physical universe is a deterministic system. So, all of our “choices” are really determined in advance. Then, what happens to our free will? Democritus: free will is an illusion!

Which do you choose?

Mint chocolate chip Vanilla with sparkles Muscle to reach for: Soul atom ?

Which do you choose? Mint chocolate chip Vanilla with sparkles Muscle to reach for: Soul atom Democritus’ determinism

Democritus’ Determinism Choice is determined. No free will? We may not be able to predict the choice in practice—because the brain is so complicated. But it could be predicted in principle.

Which do you choose? Mint chocolate chip Vanilla with sparkles Muscle to reach for: Soul atom ? Lucretius’ Indeterminism

Choice is not determined. Free will (II, ). Does the swerve save free will? Only if the atom swerves because I want it to! Otherwise my actions are now random. But random is not the same as freely chosen.

Sir John Eccles ( ) Nobel Prize, 1963 in Medicine

Which do you choose? Mint chocolate chip Vanilla with sparkles Muscle to reach for: Soul atom ? Eccles’ Indeterminism Dualism, but not violate Law of Conservation of Energy