The Newtonian interpretation of propulsion through water is based on the impulse-momentum relationship (Newton's Second Law). Basically, any change in the magnitude or direction of the water's momentum as a result of its disturbance by a swimmer's body segment(s), is a direct result of an impulse (i.e., force applied for a period of time) applied to it. Newton's Third Law dictates that there must be an equal but opposite impulse back on the swimmer's segment(s) that is then broken down into two perpendicular components, lift and drag. Drag is the force component parallel to and in the same direction as the relative fluid flow prior to segment contact. Lift is defined as the component of force perpendicular to the drag component. To use the Newtonian approach to understanding propulsion, one only has to visualize the resulting changes in magnitude and direction of the disturbed water's momentum as it moves (relatively speaking) past the swimmer's segments, and then apply Newton's Third Law. On the other hand, Bernoulli's Principle is based on the work-energy theorem that imposes serious limitations on its ability to predict lift in a practical situation such as the swimming stroke. It should be recognized that Bernoulli's Principle is not capable of predicting drag forces.