This paper shows how reactive agents can solve complex tasks without requir
ing any internal state and demonstrates that this is due to their ability t
o coordinate perception and action. By acting (i.e., by modifying their pos
ition with respect to the external environment and/or the external environm
ent itself), agents partially determine the sensory patterns they receive f
rom the environment. Agents can take advantage of this ability to: (1) sele
ct sensory patterns that are not affected by the aliasing problem and avoid
ing:those that are; (2) select sensory patterns in which groups of patterns
requiring different answers do not strongly overlap; (3) exploit the fact
that, given a certain behavior, sensory states might indirectly encode info
rmation about useful features of the environment; (4) exploit emergent beha
viors resulting from a sequence of sensory-motor loops and from the interac
tion between the robot and the environment. The relation between pure react
ive agents and pure representational agents is discussed and it is argued t
hat a large variety of intermediate cases between these two extremes exists
. In particular, attention is given to the case of agents that encode in th
eir internal states what they did in the previous portion of their lifetime
which, given a certain behavior, might indirectly encode information about
the external environment. (C) 2002 Elsevier Science B.V. All rights reserv
ed.