section_path: Philosophical history/Substantive and formal reason
section_name: Substantive and formal reason
paragraph_text: Substantive and formal reason
  
  In the formulation of Kant, who wrote some of the most influential modern treatises on the subject, the great achievement of reason (German: Vernunft) is that it is able to exercise a kind of universal law-making. Kant was able therefore to reformulate the basis of moral-practical, theoretical, and aesthetic reasoning on "universal" laws.
  
  Here, practical reasoning is the self-legislating or self-governing formulation of universal norms, and theoretical reasoning is the way humans posit universal laws of nature.
  
  Under practical reason, the moral autonomy or freedom of people depends on their ability, by the proper exercise of that reason, to behave according to laws that are given to them. This contrasted with earlier forms of morality, which depended on religious understanding and interpretation, or on nature, for their substance.
  
  According to Kant, in a free society each individual must be able to pursue their goals however they see fit, as long as their actions conform to principles given by reason. He formulated such a principle, called the " categorical imperative ", which would justify an action only if it could be universalized:
  
  Act only according to that maxim whereby you can, at the same time, will that it should become a universal law.
  
  In contrast to Hume, Kant insisted that reason itself (German Vernunft) could be used to find solutions to metaphysical problems, especially the discovery of the foundations of morality. Kant claimed that these solutions could be found with his " transcendental logic ", which unlike normal logic is not just an instrument that can be used indifferently, as it was for Aristotle, but a theoretical science in its own right and the basis of all the others.
  
  According to Jürgen Habermas, the "substantive unity" of reason has dissolved in modern times, such that it can no longer answer the question "How should I live?" Instead, the unity of reason has to be strictly formal, or "procedural". He thus described reason as a group of three autonomous spheres (on the model of Kant's three critiques):
  
  For Habermas, these three spheres are the domain of experts, and therefore need to be mediated with the " lifeworld " by philosophers. In drawing such a picture of reason, Habermas hoped to demonstrate that the substantive unity of reason, which in pre-modern societies had been able to answer questions about the good life, could be made up for by the unity of reason's formalizable procedures.