“You can recognize a deep truth by the feature that its opposite is also a deep truth,” Nobel-winning physicist Frank Wilczek observed in his beautiful meditation on complementarity as the quantum of life.

Nearly two centuries earlier, the influential Danish philosopher Søren Kierkegaard (May 5, 1813–November 11, 1855) explored the abiding metaphysical dimension of this notion in his 1843 masterwork Either/Or: A Fragment of Life (public library), which also gave us his abiding wisdom on our greatest source of unhappiness and the only true cure for our existential emptiness.

Shortly after his contemporary Charles Darwin compiled his amusing list of the pros and cons of marriage, Kierkegaard begins with this common conundrum and extrapolates from it the larger principle of how we confront our existential dilemmas:

If you marry, you will regret it; if you do not marry, you will also regret it; if you marry or if you do not marry, you will regret both; whether you marry or you do not marry, you will regret both. Laugh at the world’s follies, you will regret it; weep over them, you will also regret it; if you laugh at the world’s follies or if you weep over them, you will regret both; whether you laugh at the world’s follies or you weep over them, you will regret both. Believe a girl, you will regret it; if you do not believe her, you will also regret it; if you believe a girl or you do not believe her, you will regret both; whether you believe a girl or you do not believe her, you will regret both. If you hang yourself, you will regret it; if you do not hang yourself, you will regret it; if you hang yourself or you do not hang yourself, you will regret both; whether you hang yourself or you do not hang yourself, you will regret both. This, gentlemen, is the sum of all practical wisdom.

To get caught in this bind of “double regret,” Kierkegaard cautions, is to reduce eternity to “a painful succession of moments in time.” He writes:

It isn’t just in single moments that I view everything aeterno modo [in the mode of eternity], as Spinoza says; I am constantly aeterno modo. Many people think that’s what they are too when, having done the one or the other, they combine or mediate these opposites. But this is a misunderstanding, for the true eternity lies not behind either/or but ahead of it.

Transcending the trap of either/or, Kierkegaard suggests, is an immense expansion of possibility, which is in turn life’s greatest reward:

Were I to wish for anything I would not wish for wealth and power, but for the passion of the possible, that eye which everywhere, ever young, ever burning, sees possibility. Pleasure disappoints, not possibility.

Complement the indispensable Either/Or with Kierkegaard on boredom, the trap of busyness, the power of the minority, why haters hate, and how anxiety enhances creativity rather than hindering it, then revisit Polish poet and Nobel laureate Wislawa Szymborska’s wondrous poem “Possibilities” — the most beautiful articulation of the burden and blessing of choice ever committed to words.