Section 4b

Hume's Physics: Identity and Space for that Identity to Exist

by David Hume Icon

Hume’s Primary Qualities of the Material Universe: Space and Identity

If we remove the sounds, colours, heat, cold, and other sensible qualities of an external object, then we only are left with ‘primary qualities’.

Primary qualities are the only real ones which we have any adequate notion of. These primary qualities are space and identity, with their different mixtures and modifications; shape, motion, gravity, and cohesion.

The following are merely changes of shape and motion=

  • the generation, encrease, decay, and death of plants and animals, and
  • the operations of all bodies on each other, such as fire, light, water, air, earth, and all the elements and powers of nature.

One shape and motion produces another shape and motion.

  • This is the only principle in the material universe, either active or passive.

This system of mine has many objections. The most decisive one is that if colours, sounds, tastes, and smells are merely perceptions, then it follows that all that we conceive are not real. This means even motion, space, and solidity would not exist.

The Idea of Motion Requires a Body

Motion is a quality inconceivable alone, without a reference to some other object. The idea of motion necessarily supposes the idea of a body moving.

What is our idea of the moving body?

That idea must resolve itself into the idea of space or of identity. Consequently, the reality of motion depends on the reality of space or identity. I have=

  • proven this to be true with regard to space, and
  • shown that we can only conceive space as composed of parts endowed with colour or solidity.


The idea of space is a compound idea. But it is not compounded with an infinite number of parts or inferior ideas.

Therefore, it must finally resolve itself into an idea that is perfectly simple and indivisible.

  • These simple and indivisible parts are not ideas of space.
  • They must be non entities, unless conceived as coloured or solid.
    • Colour is excluded from any real existence.
    • Therefore, the reality of our idea of space depends on the reality of that of identity.

Space cannot be real if identity is not.


The idea of identity is the idea of two objects.

  • These two objects cannot merge their identities.
  • They maintain a separate and distinct identity.

Therefore, identity is perfectly incomprehensible alone and without the conception of bodies which=

  • are identifiable, and
  • maintain this separate and distinct existence.

What idea do we have of these bodies?

  • The ideas of colours, sounds, and other secondary qualities are excluded.
  • The idea of motion depends on the idea of space.
  • The idea of space depends on the idea of identity.

Therefore, it is impossible that the idea of identity can depend on either of them since that would be circular reasoning.

The philosophy of Descartes leaves us no satisfactory idea of identity, nor consequently of matter.

To make this more obvious, I will give a different explanation.

To create an idea of identity, we must conceive two bodies pressing on each other without any penetration.

  • This is impossible if we only have one object.
  • Two non-entities cannot exclude each other from their locations because they never possess any location nor any quality.

What idea do we create of these objects, to which we suppose identity to belong?

  • If they have identity because they are identifiable is to run on to infinity.
  • If they have identity because they are in space is either=
    • false, or
      • This means that space has inherent attributes, such as color, which is false
    • circular reasoning
      • This is because space leads back to identity

We see a moving object. We then remove its colours, sounds, smell, and temperture. We no longer see the object and subsequently, the movement also vanishes.

To restore the motion, we remove everything from the object except its identity. Thus, we spare the identity from annihilation in order to keep the object in existence.

But how can we attain this identity if we didn’t perceive the object’s colours, sounds, smell, and temperture?

Thus, we must still get all five sensory impressions so that we can get the ideas that they create in our minds that then lead us to the idea of the object’s identity.

But those sensory ideas are different from the idea of that identity. Consequently, the idea of identity, which is supposed to be real, can never be derived from any of these senses. Therefore, the feeling is the only sense remaining that can convey the impression and idea of identity.

The impression or feeling of identity is original and leads to the idea of identity. We naturally imagine that we feel the identity of bodies.

But this method of thinking is more popular than philosophical, as proven by the following reflections.

Reflection 1= Bodies are perceived through their identity. Yet the perception of identity is a different thing from the identity.

The identity and the perception of identity do not resemble each other.

A man who has a paralyzed left hand and a normal right hand on the table can perceive that his left hand is supported just as his right hand is supported by the feeling from his right hand.

An object that presses on us meets resistance. That resistance, by the motion it gives to the nerves and animal spirits, conveys a sensation to the mind.

  • But it does not follow, that that sensation is the same as that motion and resistance.

Reflection 2= The impressions of perception are simple impressions, except when considered with regard to space.

The impressions of perception do not represent identity nor any real object.

Two rocks hit each other and get impressions of the other rock on them. But neither of them creates the identity of the other rock.

A man who gets hit by a rock gets impressions from that rock, as well as the resulting identity of that rock. If we remove all feeling from the man, then he will also not be able to assign an identity to the rock.

This proves that our feelings that lead to the identity of something has no archetype or model in external objects.

Thus, identity necessarily supposes two bodies, along with contiguity and impulse.

A compound identity can never be represented by a simple impression.

  • The identity continues always invariably the same.
  • But our perceptions of it change every moment on us.
  • This is a clear proof that our impressions that lead to that identity in our minds are not representations of that identity.

Thus, there is a direct and total opposition between=

  • our reason and our senses, or
  • our conclusions from cause and effect and our conclusions on identities.

When we reason from cause and effect, we can delete all sensory impressions, but also delete the identity.