Aristotle’ s fallacy:
Aristotle, the Greek Scientist held the view that an external force is required to keep a body in uniform motion.
His concept is obsolete now, because he considered only one side of motion and fails to explain the other i. e. if body is in motion then how does it come to rest? There came the concept of the opposing external force of Friction.
For example, a ball rolled on the floor comes to rest after some time due to opposing force of friction.
Conclusion:
An external force is required to keep a body in motion, only if resistive forces( like frictional & viscous forces) are present.