The literature on time perception is discussed. This is done with reference both to the ''cognitive-timer'' model for time estimation and to the subjective experience of apparent duration. Three assumptions underlying the model are scrutinized. I stress the strong interplay among attention, arousal, and time perception, which is at the base of the cognitive-timer model. It is suggested that a multiplicative function of two key components (the number of subjective time units and their size) should predict apparent duration. Implications for (...) other cognitive domains are drawn, and in particular an analogy is suggested between apparent duration and apparent movement. (shrink)