Section 4) The Postulates of Empirical Thought in General (p. 239)

• The categories of modality do not enlarge the concept to which they are attached as predicates. They express relation of concept to faculty of knowledge.
  • Thus the principles of modality are only explanations of the concepts of possibility, actuality and necessity in their empirical employment.

First Postulate: That which agrees with the formal conditions of experience, that is, with the conditions of intuition and of concepts, is possible.

Utility of First Postulate:
• A concept may be impossible because, while containing no contradiction, it would be impossible to construct in space, i.e. two lines forming a figure.
• A concept may be groundless although free from contradiction, because it cannot be based on experience, i.e. reading other minds.

Utility of Second Postulate:
• In the mere concept of a thing, no mark of its existence is to be found.
• Postulate does not demand immediate perception, i.e. iron filings show us a magnetic field.
• Refutation of Idealism:
  • Problematic Idealism: (Descartes) there is only one empirical assertion that is certain: 'I am.'
    • Existence of objects in space is doubtful or undemonstratable.
  • Dogmatic Idealism: (Berkeley) space is impossible.
    • Unavoidable if space is interpreted as a property of things in themselves.
  • Thesis: The mere, but empirically determined, consciousness of my own existence proves the existence of objects in space outside me.
  • Proof:
    1. I am conscious of my own existence in time.
    2. All determination of time presupposes something permanent in perception.
    THEREFORE: It is only through this permanent that my existence in time can be determined.

  • Note 1: The game played by idealism has been turned against itself. Inner experience is itself possible only mediately, and only through outer experience.
  • Note 2: This thesis agrees with all our experiences. The representation 'I' is not an intuition, it is an intellectual representation of the spontaneity of the thinking person.
  • Note 3: It does not follow that every intuitive representation of outer things involves the existence of those things.

Utility of Third Postulate:
• The necessity of existence can never be known from concepts, but always only from connection with that which is perceived.
• It is not the existence of substances that we can know necessarily, only the existence of their state in accordance with the law of causality.
• Four Propositions of Nature:
  1. Nothing happens through blind chance.
  2. No necessity in nature is blind, but always conditioned and therefore intelligible necessity.
  3. Principle of Continuity forbids any leap in the series of appearances.
  4. Principle of Continuity forbids any gap in the series of appearances.
• There is not more that is possible than is actual.

• The Principles of Modality are not objectively synthetic.
  • They do not enlarge the concept to which they are applied.
  • They are subjectively synthetic.
  • They predicate the action of the faculty of knowledge through which a concept is generated.
• We may postulate the Principles of Modality since they do not increase our concept of things, but only show the manner in which it is connected to the faculty of knowledge.
  • In mathematics, a postulate cannot be proved because the procedure requires the same procedure through which we generated the concept.

• It is very noteworthy that the possibility of a thing cannot be determined from the category alone and that in order to exhibit the objective reality of the pure concept of understanding we must always have an intuition.
• For the same reason it follows that no synthetic proposition can be made from mere categories.
• It is even more noteworthy that in order to understand the possibility of things in conformity with the categories, and so to demonstrate the objective reality of the categories, we need outer intuitions.