The Membrane

When the first protocell appeared four billion years ago, its membrane barely contained the self-reproducing enzymes of the early life to create an individual unit of natural selection. Without a membrane, a better RNA molecule would benefit every other molecule in the pond equally.

Just as it created the first physical distinction between what was “self” and what was “not self,” it made the first categorical distinction: inside vs. outside.

To use this categorical distinction as a tool for organizing concepts, we need a test.

• Internal ideas are concepts that relate to what exists or happens within the boundary. If an outside observer must cross an opaque boundary to perceive it, the idea is internal.
• External ideas are concepts that relate to what exists or happens outside the boundary. If an outside observer can perceive it without crossing the boundary, the idea is external.

Relative to a given boundary, every concept, no matter how complex, can be sorted according to this most basic distinction: is it “internal” or “external” to the boundary, just as when you cut a piece of paper in two, you have a left side and a right side. There is no “third side” created by the scissors.

A book’s weight is an external concept, measurable without opening it. The narrative within requires boundary crossing: an internal idea. A person’s thoughts? Internal, hidden behind the boundary of the skull.

A conversation is internal to the room (an outsider must cross to perceive it), external to each participant at the table (they perceive it directly, no boundary to cross).

Early life didn’t stop at the fragile fatty acid shells; it needed semipermeable membranes that were selectively open, and they uncovered more categories.