Reason

===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 ({{lang-de|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 reason]]ing is the self-legislating or self-governing formulation of universal [[norm (philosophy)|norms]], and [[theory|theoretical]] reasoning is the way humans posit universal [[natural law|laws of nature]].<ref>{{multiref2
|1={{cite book|first=Immanuel|last=Kant|author-link=Immanuel Kant|title=[[Critique of Pure Reason]]|year=1781}}
|2={{cite book|first=Immanuel|last=Kant|author-link=Immanuel Kant|title=[[Critique of Practical Reason]]|year=1788}}
}}</ref>

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 [[religion|religious understanding]] and interpretation, or on [[nature]], for their substance.<ref>{{cite book|first=Michael|last=Sandel|title=Justice: What's the Right Thing to Do?|location=New York|publisher=Farrar, Straus and Giroux|year=2009}}</ref>

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:
<blockquote>Act only according to that maxim whereby you can, at the same time, will that it should become a universal law.<ref name="Ellington">{{cite book|last1=Kant|first1=Immanuel|author-link=Immanuel Kant|translator-first=James W.|translator-last=Ellington|orig-year=1785|title=Grounding for the Metaphysics of Morals|edition=3rd|publisher=Hackett|year=1993|page=[https://archive.org/details/groundingformet000kant/page/30 30]|isbn=978-0872201668|url=https://archive.org/details/groundingformet000kant}}</ref></blockquote>

In contrast to Hume, Kant insisted that reason itself (German {{lang|de|[[wikt:Vernunft|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.<ref>{{multiref2
|1={{cite book|last=Velkley|first=Richard|year=2002|chapter=On Kant's Socratism|title=Being After Rousseau|publisher=University of Chicago Press}}
|2={{cite book|first=Immanuel|last=Kant|author-link=Immanuel Kant|title=[[Critique of Pure Reason]]|year=1781|at=Preface}}
}}</ref>

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):

; Cognitive–instrumental reason: the kind of reason employed by the sciences; used to observe events, to predict and control outcomes, and to intervene in the world on the basis of its hypotheses
; Moral–practical reason: what we use to deliberate and discuss issues in the moral and political realm, according to universalizable procedures (similar to Kant's categorical imperative)
; Aesthetic reason: typically found in works of art and literature, and encompasses the novel ways of seeing the world and interpreting things that those practices embody

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.<ref>{{cite book|first=Jürgen|last=Habermas|author-link=Jürgen Habermas|title=Moral Consciousness and Communicative Action|location=Cambridge, Mass.|publisher=MIT Press|year=1995}}</ref>