But why does the baby cry? It's a response to perceptual events, thus some rational processing is going on, just not the kind we're use to defining as rational.
Perhaps then the term rational being equivocated with cognitive functioning is the problem. Consider the fact that your brain is 'thinking' everytime you flip a TV channel or walk, it's parsing all those perceptual events generated by your senses (with the help of the nervous system). It's basically 'thinking' automatically, but this type of thinking doesn't come with concepts, it comes with percepts (the integrated/composed form of perceptual events). Each percept triggers a different response depending on a number of factors: memory, genetic 'instinct', and natural response (evolutionary adaptations). So these responses could be complex, but not always so, in the case of the baby, it's simply crying. This response requires very few steps as the nervous systems and motor nerves are able to accuate each muscle required for the response, it simply 'issues' the 'command' to cry.
-- Brede