1. Margolis, E., Lawrence, S.: Concept. Stanford Encyclopedia of Philosophy. Metaphysics Research Lab at Stanford University (retrieved November 6, 2012)
2. Belohlavek, R., Klir, G.J. (eds.): Concepts and Fuzzy Logic. The MIT Press, Cambridge (2011)
3. Machery, E.: Concepts are not a Natural Kind. Phil. of Science 72, 444–467 (2005)
4. Steup, M.: Epistemology. The Stanford Encyclopedia of Philosophy (Winter 2013 Edition),
http://plato.stanford.edu/archives/win2013/entries/epistemology
5. Kant, I.: Critic of Pure Reason, ch. III. The Architectonic of Pure Reason, The Cambridge Edition of the Works of Immanuel Kant. Cambridge University Press (1998),
http://www.philosophy-index.com/kant/critique_pure_reason/ii_iii.php