You can quantity maths. It’s the prime example of something objective and quantifiable. No I’m clearly not saying that.

What I mean with “exists only in our minds” is that it isn’t some externally measurable thing. There is no moralometer which can measure the morality of an action. It only exists to us, humans. That makes it subjective.

If you’re wondering about the meaning of the previous comment it is to clarify that saying there are no objective moral truths doesn’t mean I am dismissing morality. It just means that objectivity isn’t applicable to it.