The following paper is available at http://www.math.yorku.ca/Who/Faculty/Tholen/research.html J.Rosicky and W.Tholen, Left-determined model categories and universal homotopy theories Abstract: We say that a model category is left-determined if the weak equivalences are generated (in a suitable sense) by the cofibrations. While the model category of simplicial sets is not left-determined, we show that its non-oriented variant, the category of symmetric simplicial sets (in the sense of Lawvere and Grandis) carries a natural left-determined model category structure. This is used to give another and, as we believe, simpler proof of a recent result of D. Dugger about universal homotopy theories.