In order to understand man's different and often contradictory ideologies we need to explore their deeper roots, analyzing ideas. We do not comprehend the nature of ideas and yet we use and oppose them, realizing that they are wonderful and dangerous entities whose action is comparable to that of drugs. They depress or stimulate the organism, leading men toward great deeds or toward individual and collective disaster. Ideas interfere with the autonomic mechanisms of the body, and are also projected into the social world, creating machines, institutions, methods and goals. Notwithstanding, we usually pay little attention to this tool, creator of all other tools and root of human activity. Assuredly a theory of the idea and of ideologies cannot be developed without the convergent effort of scientists and philosophers, but we can glimpse the possibility of its construction, conscious nevertheless, of our shortcomings and of the provisional character of the presented synthesis.