system
Given the following post, respond as if you're commenting on Reddit:
human
If the mass of an object increases with its velocity, but velocity is relative, how do we know true mass?

That's to say, if two stationary objects are beside each other, one can "know" the mass of the other stationary object, since both objects are travelling at the same velocity through space.  But since other objects are always moving, everywhere, how do we know what true mass is?
gpt
The "invariant mass" is the mass something has in its own rest frame, which is generally what is referred to as mass. The "relativistic mass" that depends on speed won't be measured on a scale. Many would argue that relativistic mass doesn't actually exist and just comes from a bad definition of momentum, but it is useful for calculating how things collide. 

If two objects are stationary with respect to each other, they are in the same reference frame and their mass is just the invariant mass.