The Rescorla experiment also demonstrates another problem in associative theorizing: the question of why some property is singled out as a CS as opposed to different, equally contemporaneously instantiated properties. Put a different way, one needs a principle to say what the “same situation” amounts to in generalizations such as Thorndike’s laws. For instance, if a CS and a US, say a tone and a shock, are perfectly paired so that they are either both present or both absent, the organism won’t associate the location it received shocks (e.g., the experimental setting) with getting shocked, it will just associate the tone with the shocks. But in the condition where the US occurs without the CS, but the CS does not occur without the US, the organism will gain an association between the shocks and the location. However, in both cases the location is present on every trial.[37] In contrast to shocks, x-ray radiation, when used as a US, never appears to become associated with location, even if they are always perfectly paired  (Garcia et al.  1972).[38]