Self defence remains a sticky issue.

If your home is invaded by a burglar and you injure the intruder or kill them whilst attempting to protect your family, you may be deemed a 'criminal'.

If you travel to a foreign land in the service of the military and kill many strangers in battle, you may be called a 'hero'.

Every martial artist should understand Newton's Laws of Physics.
To proceed without this knowledge is foolish.

Any martial art uses the human body.
The human body is subject to the laws of physics.

Now, watch what people are doing...
Allowing an object to continue moving, unimpeded?
F = M x A?
Force against force?

Is this really what martial artists typically are doing?

Yielding is a most misunderstood quality.

It is not giving-in, collapsing or weakness.
It is about giving the opponent room to over-reach, over-commit, miss.
It is about making space, re-positioning.

All applications contain some facet of yielding, even if the reality is simply not opposing force.

A journey of a thousand miles may indeed start with one step. But are you really going anywhere?
This is an important question.
Although your physical position may alter relative to external objects, are 'you' actually going anywhere at all?
Do you understand?

Tap your chest. You are here. Now walk across the room. Tap your chest again. You are still here.
Your body may have moved across the room, but 'you' haven't gone anywhere at all.

If you realise that your body occupies the same space at all times, then you see the form and your relationship with an opponent quite differently.
This is what 'central equilibrium' is really about.

The form is merely exploring variations of the three dimensions. You turn the waist, shift the weight, move the hands, the legs, the eyes. 
But you essentially remain exactly where you are.