The editor has asked me to reply to a paper, by G. Lochak, refuting a theorem of mine on hidden variables. If I understand correctly, Lochak finds that I failed somehow to allow for the effect on these variables of the measuring equipment. I will try to explain why I do not agree. The opportunity will also be taken here to comment on another refutation, by L. de la Peña, A. M. Cetto and T. A. Brody, and on another by L. de Broglie. Yet another refutation of the same theorem, by J. Bub, has already been refuted by S. Freedman and E. P. Wigner.

Let us recall a typical context to which the theorem is relevant. A ‘pair of spin ½ particles’ is produced in a space-time region 3 and activates counting systems, preceded by Stern–Gerlach magnets, in space–time regions 1 and 2. The system at 1 is such that one of two counters (‘up’ or ‘down’) registers each time the experiment is done; correspondingly we label the result there by A (= + 1 or – 1). Likewise the system at 2 is such that one of two counters registers each time the experiment is done, giving B (= + 1 or – 1).