Hi, I am trying to read the chapter 2 of the book "Speakable and Unspeakable in Quantum Mechanics" from J. Bell. I have got some doubts and I would be glad if someone could help me. I see that λ is a vector that makes the role of the hidden variable. All the tricks about the angles between a, a' and λ I think that I could understand. Now some doubts: The equation (1) A(a, λ) = [itex]\mp[/itex] 1 means that A may take just 1 or - 1? So is A not an average, but a measure itself ( like spin 1/2 or - 1/2), right? In the equation (4) sign λ.a' Is it means sign( λ.a' ) ? And in this case the author is just saying that A(a, λ ) = sign( λ.a' ) ? And so is it in that sense that λ would define the result of the measure? If λ.a' > 0 the measure will show the spin up, otherwise spin down. That is all for a while. I will go on reading the rest of the chapter. Thanks in advance.