Tarde denies the existence of higher-level entities (like "society" according to Durkheim). This is an atomism not just of composition, but of organization. There is no such thing as social laws and regulations, social norms, social impositions. There are only power relations among individuals. Certain individuals impose on others; certain individuals are imitated by others. Social coherence is merely the result of imitation on a mass scale, together with raw power impositions.
However, Tarde is not advocating the sort of "individualism" one sees in traditional liberalism, from Adam Smith to the free-market fanatics of today. ... Individuals, no less than human societies, are composed of multiple elements that overpower and/or imitate one another. You can't call Newton the author of the laws of motion any more than you could call 17th Century British society, or King Charles as its representative, the author of those laws. The author is more properly one particular atomistic thought in Newton's brain, a thought that overpowered the other thoughts in his brain, compelled them to obey it, or seduced them to imitate it.
By a similar argument, it cannot possibly be the case that all hydrogen atoms are uniform and interchangeable. The only explanation for the apparent uniformity of nature is that one particular hydrogen atom dominated the others, forced them to obey it, or induced them to imitate it.
The ultimate motivating forces that move all of the world, whether human beings in society, thoughts in a single brain, or hydrogen atoms in a gas, are according to Tarde belief and desire. There's nothing else. Rocks and stars, indeed atoms themselves, believe and desire just as we do. At the other extreme, things like ideologies and customs and social classes and bureaucracies can be explained merely as statistical aggregations of particular beliefs and desires, amplified by mass imitation.
Belief and desire!