et al., 1935). He therefore argued for ”local realism” and rejected the previous explanation. Bohr disagreed with Einstein and his ”local realist” assumption. Neither Einstein nor Bohr apparently realized that the hypothesis of local realism was subject to empirical test. In 1964, John Bell showed that a test was possible; he proved that if strict locality were true, there would be certain inequality relations between measurable quantities that must hold (Bell, 1964); quantum theory predicted that these inequalities must be violated. Experiments found Bell’s inequalities were indeed violated (though see discussion below for further comments). Einstein was wrong; local realism is false. We now show how results on causal interaction can be used to produce an alternative proof of Bell’s theorem. Suppose we have two particles and can use devices to measure the spin of each, along any axis of our choosing. Let X 1 and X 2 be two ”interventions” each taking values in { 0 , 1 , 2 } , where X 1 records the angle (i.e. axis in space) at which particle 1 is measured, and X 2 records the angle at which particle 2 is measured, and where 0 , 1 , 2 correspond to three particular angles. Let Y 1 ( x 1 , x 2 ) be the binary spin (up= 1 or down= − 1) of particle 1 and Y 2 ( x 1 , x 2 ) be the spin for particle 2, when particle 1 is measured at angle x 1 and particle two is measured at x 2 . In the language of the Neyman model Y$_{i}$ ( x 1 , x 2 ) is the counterfactual response of particle i under the joint intervention ( x 1 , x 2 ). Let M ( x 1 , x 2 ) = 1 { Y 1 ( x 1 , x 2 ) = Y 2 ( x 1 , x 2 ) } be an indicator function that the spin directions agree so that M ( x 1 , x 2 ) = 1 if the spin direction agree and M ( x 1 , x 2 ) = 0 if they disagree. Suppose that the particles are in a maximally entangled state. Then, according to quantum mechanics of the 2 particle system, for i, j = 0 , 1 , 2 , E [ M ( x 1 = i, x 2 = j )] = sin $^{2}$(Δ$_{ij}$ / 2), where Δ$_{ij}$ is the angle between angles i and j . This result has been confirmed by experiments in which the angles of measurement were randomized (though see discussion below for further comments). Therefore in what follows we take { E [ M ( x 1 , x 2 )]; x 1 ∈ { 0 , 1 , 2 } , x 2 ∈ { 0 , 1 , 2 }} as known, based on the data from experiment. Since sin (0) = 0, M ( i, i ) = 0 , i = 0 , 1 , 2, with 3 A Proof of Bell’s Inequality Using Causal Interactions