If we are talking on the force being exerted by a segment of a rope of lenght R on the right on a point M which is being also pulled from the Left by a segment of rope R as shown in the figure attached. Then we invoke Newton's Third Law:
"Any force exerted by an object (in this case a segment of the rope) also suffers a equal and opposite force".
If we pick
whis is the tension exerted by the right segment then the left segment will also exert an equal and opposite force so we have that 
"Any force exerted by an object (in this case a segment of the rope) also suffers a equal and opposite force".
If we pick