And that there is no conflict between the action of physical causes and final causes is demonstrated by the works of man, since they all result from the harmonious action of both. In studying human works we ask two questions,--"How?" and "Why?" We ask, "What is it for?" and "How is it done?" The two lines of inquiry run parallel, and without conflict. So, in studying the works of Nature, to seek for design does not obstruct the investigation of causes, and may often aid it. Thus Harvey is said to have been led to the discovery of the circulation of the blood by seeking for the use of the valves of the veins and heart. The human mind is so constituted that, whenever it sees an event, it is obliged to infer a cause. So, whenever it sees adaptation, it infers design. It is not necessary to know the end proposed, or who were the agents. Adaptation itself, implying the use of means, leads us irresistibly to infer intention. We do not know who built Stonehenge, or some of the pyramids, or what they were built for; but no one doubts that they were the result of design. This inference is strengthened if we see combination toward an end, and preparation made beforehand for a result which comes afterward. From preparation, combination, and adaptation, we are led to believe in the presence of human design even where we did not before know of the presence of human beings. A few rudely shaped stones, found in a stratum belonging to the Quaternary period, in which man had before not been believed to exist, changed that opinion. Those chipped flints showed adaptation; from adaptation design was inferred; and design implied the presence of man. Now, we find in Nature, especially in the organization and instincts of animals, myriads of similar instances of preparation, combination, and adaptation. Two explanations only of this occurred to antiquity,--design and chance. Socrates, Plato, and others, were led by such facts to infer the creation of the world by an intelligent author--"ille opifex rerum." Democritus, Epicurus, and Lucretius, ascribed it to the fortuitous concourse of atoms. But modern science has expelled chance from the universe, and substituted law. Laplace, observing forty-three instances in the solar system of planets and their satellites revolving on their axes or moving in their orbits, from west to east, declared that this could not be a mere coincidence. Chance, therefore, being set aside, the question takes another form: "Did the cosmos that we see come by design or by law?"