The intellectual foundation of the Enlightenment was as small and simple as its repercussions have been vast and complex. The world (and everything it is composed of) is susceptible to rational investigation. Perhaps this does not seem particularly radical to you. That we are able to examine things in the world, experiment upon them, reflect on the way that they behave and draw conclusions from this may seem to be essential to the human condition. How else did we ever get from seeing fire to making fire to building steam engines? How did we get from cracking someone over the head with a rock to the wergild to the criminal justice system? Surely human being are innately rational creatures, applying their understanding to experience and drawing conclusions?
On one level this is view of humanity is undeniable. All of us make rational decisions influenced by evidence every single day. I look out the window and pick up an umbrella, I look at my change and put back the crisps. These are self-evidently rational decisions, and we are the only species on the planet with the capability of making them. But this is the point where our view of man runs into difficulties. Innately rational animals though we are, we cannot escape our animal natures. We do this unique thing with our brains, but it was not all our brains are constructed to do.
Autonomic functions are vital of course, but we are also equipped to do a whole other range of things non-rationally. We judge distance, differentiate colour, brace ourselves for landing, experience pleasure and pain and respond to them too, we catch balls and dodge blows, we recognise faces and language. Of course, there is a rational element to most of these too. Maybe I only catch the ball because I want to get the batsman out, for example. And of course the question of the instinct for language, although empirically well-supported, raises fundamental and profoundly difficult questions about the "wiring" of our brains. The line between the rational and the non-rational suddenly looks very blurred, perhaps logically undefinable, in the same way as the exact number of hairs I would need to pluck from my head to consider myself bald is, or perhaps even non-existent as a line, with elements of rationality and irrationality included in all human behaviour.
Along these lines, I tend to believe that the reason animal behaviour so often seems like our own rational behaviour is less that animals share traits of our rationality, and more that we share elements of their non-rationality. The dog begging for food resembles me begging for money, not because the dog's behaviour has some element of rationality in it, but because my behaviour has so many non-rational elements.
All this is a way of explaining why we should not be surprised either at the existence and extent of Enlightenment project, or at the extent to which it is unfulfilled. To indulge in a little paradox: "Dog like man" is not true, even though "man like dog" is. Our approach to the world, and the rationality that is inherent in it, is grounded in the functioning of brains that necessarily make use of non-rational techniques and components, even to the extent that eliminating these components eliminates the possibility of rationality.