Are mass or inertia the same thing, the same properties?

Question

I think so. But I wold like to know another opinions to confirm.

Answer 1

Yes, but. I would say that mass is the property that underlies inertia - And also underlies gravity. I'm not sure if these distinctions matter to you.

Mass is the amount of stuff there is. It's a property of the object. Post-Einstein, it's the rest-energy of the stuff, rather than its "amount".

It so-happens that inertial mass, the property of matter that resists change in velocity (the $m$ in $F=ma$) is equal to this amount. It didn't have to be this way, philosophically, the two concepts are different. So as a matter of empirical fact we found that mass is the property of matter that resists changing velocity, i.e. it is the property that underlies inertia.

At the same time, we also found that mass is the property of matter that underlies gravity. Again, this needen't be the case philosophically. It's not like that the idea of "amount of rest energy" or "amount of stuff" is the same as the idea of "what causes gravity". But in practice, the same thing is both.