Whose Force Is It?

I have been busy with work and moving house lately, so I haven't been able to write much.

Anyway, one of the questions that came up during my taiji journey is, how do you know whether it is you using force, or is it your opponent using force? Whose force is it anyway?

What I heard from my teacher is that, sometimes, when you feel that your opponent is using a lot of force, it may not be so. It could be that you are using brute force, and because your opponent is able to relax and return your force to you, you think that he is the one using force. When in actual fact, the source of the force is from you.

So how do you know whose force is it? Well, if you are using brute force, your arms will start to tire soon. If your opponent is the one using brute force and you are relaxed, then his arms will start to tire, not yours.