The permanent strand in philosophy that attempts to tie knowledge to experience. Experience is thought of either as the sensory contents of consciousness, or as whatever is expressed in some designated class of statements that can be observed to be true by the use of the senses. Empiricism denies that there is any knowledge outside this class, or at least outside whatever is given by legitimate theorizing on the basis of this class. It may take the form of denying that there is any a priori knowledge, or knowledge of necessary truths, or any innate or intuitive knowledge or general principles gaining credibility simply through the use of reason; it is thus principally contrasted with rationalism. An empiricist account of our concepts will hold that they depend upon experience: ‘nihil in intellectu nisi prius in sensu’ (nothing in the intellect that was not previously in the senses). Some philosophers such as Aquinas have held this principle without denying that reason can apprehend knowledge, provided it uses the materials afforded by the senses. One of the main problems for empiricism is to accommodate the way in which thought does not just derive from experience, but provides us with categories with which to organize it. The necessity for such addition (and its legitimacy) is the central theme of the Critique of Pure Reason of Kant. Radical empiricism, as espoused by James, holds that the relations between experiences that are implied in their organization are themselves objects of observation. The key problems for empiricism include avoiding a picture according to which the subject knows nothing but experiences of the present moment (scepticism), demarcating the legitimate basis of theory in observation, defending the view that observation is itself free of non-empirical elements, describing legitimate ways of using observation in building a picture of the world, and explaining or explaining away knowledge that appears to have no basis in experience, notably mathematical, logical, or other a priori knowledge. See also myth of the given.