  We study the way the unitary evolution of spin 1/2 particules can be
represented in a counterfactual definiteness setting. More precisely, by
representing the state of such a particule by a triplet of values corresponding
to the supposedly pre-existing outcomes of some measurements (those
corresponding to the three Pauli matrices), we analyse the evolution of our
representation when some unitary gates (namely, the Hadamard gate, the \pi/2
phase shifter and the controlled-not) are applied. Then, we describe in terms
of triplets the creation of an EPR pair and discuss the possibility of having
this representation comply with the predictions of quantum mechanics. Finally,
we show that this is not possible unless one of the assumptions used to build
our model is dropped.
