Explain how does Newton’s first law of motion give us the definition of force?

Newton’s first law of motion states that an external force must be applied to change the state of rest or uniform motion in a straight line. This means that force applied on a body alone, can change it's state of rest or uniform motion. 
For e.g., force is not able to change the state e.g. if we push the wall it will not change the state of rest.
Hence force may be defined as the external agency that changes or tends to change the state of rest or uniform motion of a body in a straight line.