Themes of The Essays by Michel de Montaigne include human nature, truth, and proper behavior.

Human Nature

Montaigne believes that people are only alive "by chance" and that "no one lays down a certain design for his life." He also says that, if human nature was easily discerned, then a brave man would always be brave: he wouldn't charge at the enemy lines on the battlefield and then cry over losing a court case, for example. Montaigne therefore argues that human nature is transient and no one is fixated on a particular end. He says that human nature is difficult to grasp because it's constantly moving like water through a palm. He says:

The more you clutch your hand to squeeze and hold what is in its own nature flowing so much the more you lose what you would grasp and hold.

To Montaigne, human nature is difficult to comprehend.

Truth

In "Of Experience," Montaigne says that:

There is no desire more natural than that of knowledge

This is the perhaps the underlying purpose for his essays. He's searching for knowledge of the truth of the world—though he rarely reaches a firm conclusion. The majority of his essays are an argument between a variety of concepts without a judgment on which is perfectly right.

It's not that Montaigne doesn't have conviction in his opinions; he does, and he states what he believes frequently. But some deeper concepts are things he doesn't believe can be comprehended by man. One difficulty that he mentions in "Of Liars" is that truth is only one thing but a lie can be a thousand things—which makes it even more difficult to discern the truth.

Proper Behavior

Taken together, the The Essays provide a framework for how a person should attempt to live their best life. Montaigne gives a series of guidelines that can help a person improve as they attempt to navigate through some of the most universal human experiences.

Some of the advice is practical in nature; for example, he says that people shouldn't purge, because he finds it very unhealthy. Some of it is more philosophical. For example, he says in "Of Drunkenness" that all vices are alike. However, he goes on to say that drunkeness is "a gross and brutish vice," which makes it obvious that he doesn't believe it's proper for men to get drunk.