Newton's First Law: The law of inertia

An object will remain in its state of motion unless acted on by an external force

This law applies to "stationary" objects as well as moving ones. An object with a velocity of zero still has a velocity. What is often missed in this law is that it also states that an object will remain in its state of motion if all the forces on it balance out.

The important consideration here is that there may be multiple forces acting but the sum of these different forces sum to zero.

The ball, above, is not accelerating. Therefore, there are no net forces acting on the ball. There are two forces acting; the weight force pulling the ball down and the tension force pulling the ball up. Note that while these forces are equal in magnitude and opposite in direction this is not an example of Newton's third law. The reason for this is that the two forces are not the same type of force. One is a gravitational force while the other is a tension force.

The correct way to analyse this system is to examine the sum of all forces and set the sum to zero.

Using the convention that up is positive.

From this we can determine that T and mg have the save value and opposite directions but this does not make them an action / reaction pair.