Before I go further I need to elaborate somewhat the very brief distinction which I have already made between a signal and an index (above, p. 12). You should again keep a watch on Fig. 1 (p. 12).

Signal refers to any automatic trigger response mechanism. In nature, most signals are biological. Every natural species has become adapted by evolution to respond to its environment by a complex mesh of signals. To take a simple human example. If I do not drink for several hours on a hot day I become thirsty. ‘Feeling thirsty’ is a biological signal which triggers off a response. I look for something to drink.

The extent to which communication between adult human individuals is governed by true signals is not at all clear, but a mother's responses to the cries and smiles of her infant are certainly largely ‘instinctive’, and we do not shed our animal nature as we get older.

Two general characteristics of signals deserve particular attention:

1. A signal is always part of a cause and effect sequence. It is first generated by a prior cause and then functions as a cause to generate a later effect.

2. There is always a time lag between a signal and its consequence.

Human technical actions which alter the physical state of the world out-there (p. 9) closely resemble signals in this respect. The principal difference is that signals are ‘automatic’ in that they do not entail an intentional response on the part of the receiver, but they are not fully mechanical in that the effectiveness of the signal depends upon the sense response of the receiver which is not fully predictable.