Kant’s Critique of Pure Reason

I Kant’s Critique of Pure Reason, Ed. 2 1787.

This is hard going. I’ve done my best. Kant himself provided a more accessible account of his ideas in his Prolegomena, which also see.


Synthetic a priori

Kant develops a theory of a priori reason and hence of perception. He extensively catalogues different types of reasoning, showing the need for ‘synthetic a priori’ reasoning, as in mathematics, as well as analytic reasoning.

Space and time

Kant’s notion is that one has a priori intuitions of space and time that we fit sensations to, to make sense of them.  Thus, without assuming anything about a possible external reality we can reason about our possible perceptions.


Summarising, Kant supposes that everything that ‘is’ is a result of some activity, and that the effect of all activities is conditional, as may be the fact of an activity. These conditions are thus the ’cause’ of what is, as an effect. But what ‘is’ as a cause ‘is’, and so is as an effect. Hence one must have an indefinite backward chain of cause and effect. But the associated times do not necessarily need to go back indefinitely in time: they may be bounded. Thus there may be nothing of a chain at one time, but then at a later time the chain may be active, but without a particular beginning.


Kant shows how reasoning is typically a ‘work in progress’, which goes wrong when it claims a certainty or finality that is not justified. For example, he distinguishes between saying that a chain of causality goes back indefinitely, meaning however far you go back there may always be further to go, and saying that chains go back infinitely, implying that the universe had no ‘big bang’. He is equally critical of dogmatism, misplaced ‘mathematical’ (i.e. formal) and ‘practical’ or ‘pragmatic’ reasoning.


Kant’s 12 ‘pure concepts of the understanding which apply to objects of intuition’ include ‘Of Community (reciprocity between agent and patient (sic))’.  ‘This is a quite different kind of connection from that which is found in the mere relation of cause and effect’ and plays a key role in Kant’s concept of a ‘whole’. Most of Kant’s examples of the mis-use of reason are when reasoning has been confined to mundane objects and causality, but leaps to make groundless deductions about community.

The signs of a community may fade as one looks back in time, such that the community did not exist at one time but came into being later, with no linear cause and effect chain.

The forms of possible knowledge

Kant has a corresponding notion of the structure of possible knowledge. This includes partial orders for space and time, of bounded, nested and inter-related communities, wholes and systems whose ‘ways of existing’ may persist or periodically ‘alter’

Kant also distinguishes between scientific and technical reasoning. The former is always uncertain and striving, whereas the latter may be pragmatic. Kant made a significant contribution to modern notions of science.

Kant argues for a practical reason in which one seeks ‘to be worthy to be happy’, rather than directly to be happy, thus avoiding some difficulties of the direct approach. His approach to decision-making is to maximize utility unless some categorical imperative is at stake.



Kant’s notion of the relationships of ‘community’ clearly generates complexity beyond the linear cause-effect relationships. Much of the work is about the inappropriateness of methods of reasoning developed for linear situations in complex cases.


Kant’s Critique does not deal with uncertainty or probability explicitly, beyond lack of certainty. However in so far as theories of uncertainty and probabilities are ways of reasoning, Kant’s recommendations apply to them.

In Bayesian probability theory a posteriori probability is derived from likelihoods and a priori probabilities. These a priori probabilities are sometimes empirical (derived from statistics), sometimes a priori analytic (as when the notion of a ‘fair coin’ entails the probability of ‘Heads’) and sometimes a priori synthetic (as when applying a ‘rule indifference’). The critique thoroughly analyses a priori synthetic propositions, claiming to describe all valid ones. The rest he relegates to ‘practical reason’, about which he is damning. The critique thus presents a case against the notion that there is a valid method for assigning a priori probabilities.

Practical reason

Practical people like to act as if they knew what the situation was, which they can only do by ignoring the inevitable uncertainties. They are typically pragmatic, in that they will convince themselves that the conditions that are necessary for their action to succeed are the case, and continue to believe it for as long as possible.

Sense Making

The critique is about the reasoning part of sense making. If sense making is seen as finding a frame within which to interpret the perceptions, then Kant regards this as ‘practical’. But Kant notes that in sense-making the presence of some things may make others things more probable, so that perceptions form complexes. Thus there is a reflexive relationship between the choice of frame and the contents. Kant’s ‘pure reason’ has regard to this complexity and the resultant uncertainty. Thus he would regard sense-making as folly when it makes more certain sense than is justified, neglecting the complexities and uncertainties. What one needs is to be able to have regard for all the conditional possibilities and to be able to ‘take a risk’ without repressing any possibilities.

Free will

The departure point for Kant’s more metaphysical ruminations is the consideration of free will. According to conventional reason free will is incompatible with law-like causality, but Kant’s concept of community (complexity) in effect creates gaps in conventional theories for free will to operate, without it being possible to prove that it does so. But a belief in the possibility of free will seems useful.


Kant’s notion of morality is to strive to be worthy of happiness. This presumably includes working towards establishing protocols for dispute resolution, and other useful conventions.

The Enlightenment

Kant played a full role in The Enlightenment or ‘age of reason’. He was advocating an advance on the previously conventional ‘practical reason’, to take account of complexity and uncertainty. Today the distinction is not widely appreciated, so that the Enlightenment is sometimes associated with that limited reason which it tried to transcend, and hence with a narrow rationality and scientism.

Kant’s Copernican revolution

Before Kant, the term ‘subjective’ referred to a supposed ‘thing in itself’, whereas ‘objective’ referred to how things appear to us, consistent with the subject-object distinction in grammar. Kant’s work led to a revolution in usage, in which (based on C.O.D.) one has:

  • subjective: concerning the conscious, thinking or perceiving subject
  • objective: concerning the external object presented to the subject.

But Kant’s point was the subjective ‘thing in itself’ was essentially un-knowable, whereas we could objectively known how we made sense of things (pgs 126,  139). For example, we can objectively know what we mean by a coin’s being fair, and how we would test for fairness, but we can only subjectively believe that a particular coin is actually fair. Similarly, I may know what my mental model of a fox is (and hence how I recognize one) but still have no idea of what a fox is ‘in itself’. But Kant seems to have been misunderstood, leading to some confusion.

Suppose that there are two alternative theories that one tests with an experiment. In Kant’s usage the conceptualisation of the results, such as ‘x is proportional to y within the bounds of experimental error’, can be objective, whereas the theory that is thereby supported is only subjective (there may be alternatives). Thus what is objective is ‘possible proportionality of the results of the experiment’, what is subjective is the interpretation. This fits with grammar if we say ‘the theory is supported by the results’. In common sense (and more generally prior to Kant), though, when we look at a fox it is the fox that has an objective existence and our perception of it that is subjective. Mixing Kantian terminology with this common sense view leads to confusion.

Links to others

Berkeley, Boole, Whitehead, Keynes, Smuts, Turing.


The concept of what he calls community and what we may call complexity is central to Kant’s critique of practical reason. Complexes may have obscure origins, in ‘critical instabilities’, seeming to be self-organising and ’emergent’, so that although cause-and-effect chains can be traced back indefinitely, this does not mean that they can be traced back indefinitely ‘in time’ or that there is always some prior ‘state’ that constitutes a manifest ‘objective’ cause in the sense that common sense seems to expect. This can make ‘pragmatic reason’, based on classical understandings, inappropriate and flawed. Rather, one needs to understand the logic (or logics) of what Kant calls ‘communities’.

David Marsay

4 Responses to Kant’s Critique of Pure Reason

  1. Pingback: Critique of Pure Reason | djmarsay

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