The one thing you need to know for a good life: A is A.

Aristotle’s famous law of identity states that “A is A” — that is, a thing is identical to itself. While this may seem fairly trivial, it’s actually pretty profound in a couple of fundamental ways:

• Once identity is established (that is, we confirm that two things are identical), then all the attributes of that thing are also attributes of the other, identical thing and may even share attributes with things that are similar in important respects (this particular copy of Hemingway’s For Whom the Bell Tolls is identical to itself, but it shares many important attributes with other copies of the novel, most critically the text, for example).
• Everything has its own unique nature, and even though many attributes may be shared with similar things, that one thing is identical only to itself — if something else shares exactly all the attributes of that thing, then they are by definition the same thing, not two different things. For us humans, it means we are unique and have our own specific nature: even though we share many attributes with other humans, each one of us is identical only to ourselves and therefore unique.

The 18th century Irish philosopher, Bishop Berkeley, stated it a little more exclusively: “a thing is what it is, and no other thing.” For my purpose here in talking about us humans, each one of is what we are — and not somebody else. Even if a shared “human nature” means that we share many attributes and over 99.9% of our genetic material with other humans, we each have a unique individual nature that we are yearning to express fully in the world — this is what Spinoza meant by “conatus,” the innate tendency that every living thing has to continue to exist and to enhance itself.

This may seem at odds with another fairly obvious but profound philosophical maxim: nothing ever stays the same. Heraclitus famously said nobody ever steps in the same river twice — the water is always flowing, always changing. So, how can something be identical to itself if it’s always changing? I’m obviously not the same person I was ten years ago (nobody is!), and not even the same person I was yesterday or even ten minutes ago. Isn’t this a contradiction?

To resolve this conundrum, let’s turn to Eckhart Tolle, the German-born spiritual teacher. In The Power of Now, he reviews…