Abstract: In this paper uncertainty is defined as non-uniqueness of projection of a preference on consequences onto the set of preferences on acts in a choice problem. Such definition of uncertainty seems natural, but it is not very well known despite being compatible with M. Allais principle of internal coherence [Econometrica 21(4) (1953), 503–546]. It is shown that absence of arbitrage opportunity, in the sense of the Arrow–Debreu model of securities market with a risk-less asset, represents an example and a particular case of existence of uncertainty, defined in the above sense.