 The inference model may be plausible enough when we are dealing with distant events, but rather less so for much more recent ones. In addition, the model posits a rather complex cognitive operation that is unlikely to occur in non-human animals, such as the rat. Rats, however, are rather good at measuring time over short intervals of up to a minute, as demonstrated by instrumental conditioning experiments involving the ‘free operant procedure’. In this, a given response (such as depressing a lever) will delay the occurrence of an electric shock by a fixed period of time, such as 40 seconds, described as the R-S (response-shock) interval. Eventually, rate of responding tracks the R-S interval, so that the probability of responding increases rapidly as the end of the interval approaches. (See Mackintosh 1983 for a discussion of this and related experiments.) It is hard to avoid the inference here that the mere passage of time itself is acting as a conditioned stimulus: that the rats, to put it in more anthropocentric terms, are successfully estimating intervals of time. In this case, the strength model seems more appropriate than the inference model.