Many of the activities of living things seem purposive, that is goal-directed: The needs of living things - food, shelter, sex - set up ends that they then seek to satisfy or realize.

And indeed many of the activities of human beings are purposive, and based on plans, theories, assumptions etc. that nearly always involve language or other means of symbolic representation like mathematics, diagrams or maps.

One problem here is that many of the goals and the ways to attain them that animals seem to use also seem to be too complicated for them to reason out consciously. One example are the webs spiders weave; another the hexagonal cells bees make to store honey.

Four partial explanations of purposive behavior are these:

Darwinian: Much of the behavior (and forms, limbs, capacities) of animals have slowly evolved by trial and error through the generations of animals, each of which was a unique individual with some possibly unique talent it could transmit to its children if and when it lived long enough to produce offspring. (See: Darwin)

Mathematical: Some of the things animals do, including the making of hexagonal cells by bees, can be explained by fairly simple mathematics. Thus, the shapes of the cells that bees make have been already explained mathematically in the 18th Century. (See D'Arcy Thompson).

Cybernetical: Some goal-directed activity can be explained mathematically in terms of predictions and adjustments. This originated in the 2nd World War when the mathematician Norbert Wiener created the theory that makes it easier to shoot down moving targets like airplanes, by tracking and predicting their paths by making successive approximations, adjustments and interpolations. (See: Wiener)

Neural nets: Some goal-directed activities can be explained in terms of neural nets, which are representations of neurons, that use combinations of statistics and cybernetics to represent and predict complicated processes.

It should be noted that, while the higher mammals (e.g. beavers), birds (e.g. when nesting), spiders (weaving nets), and insects (ants, bees), are capable of some very amazing goal-directed highly complicated behaviour, that so far has not even the start of a decent explanation - and humanlike artificial intelligence at present seems a rather ludicrous idea when one considers the capacities of, say, spiders - the human animal is special, and sui generis, in being capable of using complicated symbolism and language and mathematics.

This makes the sense of "purposive" (and related terms) quite different when speaking of humans or when speaking of other animals: Human beings are capable of far more refined, precise, extensive purposive planning, having the gifts of language and mathematics, than any other kind of animal.

And in the same vein it should be remarked that what seems "purposive" in computers is not due to computers (so far, at least) but to human programmers (and also to human minds being inclined to look at things that seem to move by themselves as somehow purposive).

Also, it should be noted that there are various arguments in philosophy and theology relating to purpose. Many of these are not clear, and one of the many achievements of Darwin is to have shown in principle how evolution of species is possible without assuming any purposes whatsoever, essentially on the basis of assuming (1) all animals pass on their capacities to their offspring, that are all like their parents but unique and (2) the natural environment determines which animals survive and procreate, and which don't.