In One Flew Over The Cuckoo's Nest, Kesey is primarily exploring the eternal conflict between the individual and the collective. Throughout human history, societies have often struggled to reconcile individual freedom with the safety and security of the community as a whole. Individuals must have their freedom, but how much? And who decides how much? Society needs to have its rules and regulations in order to function properly. But if those rules are too draconian or too strictly applied, then the freedom of the individual is compromised.

That's what appears to happen in the psychiatric hospital in One Flew Over The Cuckoo's Nest. Nurse Ratched, the authority figure, is concerned that her patients will undermine the proper functioning of the institution if they're given too much freedom. That's why McMurphy represents such a danger, not just to her personal authority, but to the power that the hospital exerts over its patients. McMurphy is constantly testing Nurse Ratched, seeing how far he can push the boundaries of her control. As such, he represents the human spirit, yearning to be free, chafing against the shackles imposed upon it by a society that struggles to reconcile good order with the expression of individuality.

Eventually, Nurse Ratched prevails in her battle of wills with McMurphy, at the cost of stripping him of every last vestige of humanity. But the human spirit lives on; the torch has been passed to the Chief, who in his killing of McMurphy and his subsequent escape, reasserts in the starkest possible terms the freedom of the individual to control his own destiny.

In One Flew Over the Cuckoo's Nest, there is the examination of man's struggle for individual freedom and the establishment of his sanity.

Statement #1  Man must struggle for his individuality in a conformist society that suppresses personal expression as a means of maintaining order.

In his essay "Self-Reliance," Ralph Walso Emerson writes,

Society everywhere is in conspiracy against the manhood of every one of its members. Society is a joint-stock company in which the members agree for the better securing of his bread to each shareholder, to surrender the liberty and culture of the eater.

Kesey concurs with this judgment of society, for in his novel the authority of society, represented by the "Big Nurse," dehumanizes the men in the ward.  Alienated from other Native Americans, Bromden withdraws into feigned deafness and the "fog" rather than be subjected to her dominance.  Harding, who has willingly committed himself further "surrenders his liberty" by allowing Nurse Ratched to make him "rabbit-souled" and intimidate, humiliate, and institutionalize him. 

Statement #2  One man's sanity may differ from that of another; there is no single definition for self-realization.

In one of her insightful poems, Emily Dickinson writes,

Much Madness is divinest Sense -

To a discerning Eye -

Much Sense - the starkest Madness -

’Tis the Majority

In this, as all, prevail -

Assent - and you are sane -

Demur - you’re straightway dangerous -

And handled with a Chain -