I will present a version of Kant’s refutation of idealism. My version has a different structure than Kant’s, but my goal was to make the argument clearer and less dependent on Kant’s other arguments. I conclude that, while the argument proves that we must have a concept of the external world, it does not prove that an external world truly exists. Do you think this is right? Am I being faithful to Kant?

The basic idea is that we require a concept of the external world in order to discern a temporal order among our perceptions.

I’ll break the argument down into two sub arguments and a final conclusion. The first sub argument establishes that something other than perception is required to temporally order experience. The second argues that the concept of causality is responsible for the temporal order of experience and that causality must apply to objects external to perception. The final conclusion is that an external world exists consisting of objects external to our perceptions.

Premise 1: Some experiences consist of an ordered series of moments (let’s call them temporal experiences), and the order of these moments is intrinsic to the experience.
Premise 2: Perceptions have no intrinsic temporal order.
Conclusion: Therefore, something in addition to perception is required to explain the order of moments in a temporal experience.

Let’s look at an example experience to illustrate the argument: I see a leaf fall from a tree. Now I’ll justify each step, starting with premise 1. This experience is temporal because it consists of a number of moments in a particular order: the leaf begins on a branch then descends through the air then lands on the ground. If the order of moments were reversed, I’d see the leaf rise. If the moments were scrambled, I’d see the leaf teleport around. Thus, the nature of this experience (and other temporal experiences) depends not only on the perceptions it contains but on their order. If the order of moments were different, the nature of the experience would be different. Therefore, the order of moments is intrinsic to the experience.

Here’s the justification for Premise 2: I can imagine my perceptions occurring in any order. I can imagine the leaf rising or teleporting instead of falling. If wore a VR headset, I could perceive the leaf rising or teleporting just as realistically as I saw it falling. Now my perceptions of the leaf at various points in the air happened to occur in a particular order in my experience of seeing the leaf fall. But this order is not intrinsic to the perceptions themselves.

The above argument is not a formal syllogism, so I’ll fill in the missing steps here. Take this premise: If x has a property y, and x contains a set of elements z, the fact that x contains z does not explain why x has y unless z has y. This seems to make. If a wall is red, and the wall contains bricks, the fact that the wall contains bricks does not explain why the wall is red unless the bricks are red. Now let’s apply this to the argument at hand. If an experience has an intrinsic temporal order, and the experience contains of a set perceptions, then the fact that the experience contains these perceptions does not explain why it has this temporal order unless the perceptions have this intrinsic temporal order. But as premise 2 established that perceptions have no intrinsic temporal order. Thus, the perceptions which an experience contains do not explain the order in which they occur. Therefore, if the temporal order of perceptions has an explanation, the experience must contain something other than perception.

One might respond by questioning why we need an explanation. Perhaps it is self evident that my perceptions occurred in the order that they did, and I need no explanation. But this is false. If I saw the leaf fall, then the leaf is no longer falling, so I can only know that I saw it fall if I remember having seen it fall. But my memory of having seen the leaf fall is also a temporal experience, and I can only experience this memory one moment at a time. I cannot imagine all the moments contained in the fall of the leaf next to one another and perceive their temporal order immediately. Since I can only perceive the moments in a temporal experience one moment at a time, I can never perceive the temporal order of the moments. At best, I can perceive one moment giving way to the next moment, but never a whole series containing many moments. I would not be able to comprehend the order of the moments in my experience if my experience did not contain something in addition to perception. This is true whether I experience a temporal experience presently or experience a memory of a past temporal experience.

Here’s the second sub argument
Premise 3: The order of moments in temporal experience is explained by the concept of causality.
Premise 4: Causality does not apply to perceptions.
Conclusion: Therefore, causal principles must apply to objects external to perception.

Premise 3 makes sense because causality determines the order temporal of moments. I know the leaf on the branch preceded the leaf in the air because the causal order of nature, as I conceive it, precludes the possibility that the leaf floated or teleported up and reattached itself to the branch. If there were something else besides the concept of causality to explain the order of moments in an experience, then it would be unnecessary. We could follow Hume and suppose that the concept of causality is confused. But since perceptions cannot explain temporal order, and nothing else seems to do so, it seems necessary to suppose that I really do conceive of causal principles and use them to temporally order my perceptions. Importantly, these causal concepts are not themselves perceptions. They order perceptions within an experience, but they themselves are of a different nature. Whereas perceptions consist of sensory information, concepts like causal principles seems to consist of an intelligible structure. How I come to understand specific causal principles is another matter.

Premise 4 is basically the same as premise two. Perceptions can occur in any order, and no causal principle necessitates that one perception follow upon another. The conclusion follows from these two premises straightforwardly.

The final conclusion is that there must be an external world. This world consists of the objects external to our perceptions. According to Kant, we represent these external objects to ourselves using the concept of substance. We conceive of our perceptions as corresponding to the accidents of these external substances. So the matter of the leaf would be a substance moving in space, and the colors by which we perceive the leaf would correspond to the leafs accidents.

To sum up the argument, the concept of causality is necessary to explain temporal order, and the concept of external objects is necessary to explain causality.

Kant argues that the external objects we represent to ourselves need not be at all similar to the external objects which actually exist. The leaf we conceive may be totally different than the-leaf-in-itself. But it seems to me that Kant has failed to prove that external objects actually exist. I think he’s succeeded in proving that we must have concepts of external objects in order to temporally order our experiences, but just because we have concepts of them doesn’t mean they exist. Kant seems to argue that the concepts we employ are mere tools which we use to make our perceptions intelligible. He argues that truth is the agreement between concepts and their objects. So it may be technically and trivially true, according to this definition, that there is an external world, because the concept of substance agrees with the perceptual objects to which we apply it. However, this doesn’t mean that there really are things existing outside and independently of us. Our concept of an external world agrees with perceptions, but it may not correspond to an actual external world.