Is it "a second nature" or "second nature"?

Question

I wanted to say,

Driving a car is like a second nature to me.

But Google Docs and ChatGPT both corrected my sentence to:

Driving a car is like second nature to me.

So which is correct, and why do we remove the "a", but use the "a" if it is:

Detecting a scam is like a natural instinct to me.

Answer 1

I wanted to say,

Driving a car is like a second nature to me.

But Google Docs and ChatGPT both corrected my sentence to:

Driving a car is like second nature to me.

So which is correct, and why do we remove the "a", but use the "a" if it is:

Detecting a scam is like a natural instinct to me.

What exactly is "a second nature" or "a first nature?" It's more just an expression people use. The expression is "second nature" with no preceding article, though it isn't that wrong to include the article. However, there is no "like." If you add "like" then it sounds like you need "a" which it seems like you're picking up on. But

Driving a car is like [a] second nature to me.

should be

Driving a car is second nature to me.

It is similar to:

Detecting a scam is natural instinct to me.

(Note: natural instinct isn't an expression, so I wouldn't use the above sentence.)

Answer 2

I wanted to say,

Driving a car is like a second nature to me.

But Google Docs and ChatGPT both corrected my sentence to:

Driving a car is like second nature to me.

So which is correct, and why do we remove the "a", but use the "a" if it is:

Detecting a scam is like a natural instinct to me.

Just remove like. Second nature is an expression. Natural instinct is an actual thing you're describing which is not an expression. The rules of the expression say to not use like or a:

Driving a car is second nature to me.

There is no such rules for the non-expression "natural instinct:"

Detecting a scam is like a natural instinct to me.

Detecting a scam is a natural instinct to me.

Detecting a scam is natural instinct to me.