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Newton, Euler equation and Navier-Stokes equation of fluid mechanics (1)

Newton said in preface of his Principia that rational mechanics will be the science of motions resulting from any forces whatsoever, and of the forces required to produce any motions, accurately proposed and demonstrated. He said that for all the difficulty of philosophy seems to consist in this–from the phenomena of motions to investigate the forces of the nature, and then from these forces to demonstrate the other phenomena; and to this end the general propositions in the first and second book are directed. In the third book we give an example of this in the explication of the system of the world; for by the propositions mathematically demonstrated in the former books, we in the third derive from celestial phenomena the forces of gravity with which bodies tend to the sun and several planets. Then from these forces, by other propositions which are also mathematical, we deduce the motions of the planets, the comets, the moon, and the sea. I wish we could derive the rest of the phenomena of nature by the same kind of reasoning from mechanical principles.

Newton said that for all the difficulty of philosophy seems to consist in this–from the phenomena of motions to investigate the forces of the nature, and then from these forces to demonstrate the other phenomena. In fact, Newton’s main idea, or his principle of philosophy consists in this–from the phenomena of motions to investigate the forces of the nature, and then from these forces to demonstrate the other phenomena. Newton needs mathematical tools, when he investigates forces from the phenomena of motions and deduces other phenomena from these forces; so Newton writes book Ⅰand book Ⅱ; these two books are just mathematical tools. Book Ⅲ demonstrates Newton’s main idea or principle of philosophy–from the phenomena of motions to investigate the forces of the nature, and then from these forces to demonstrate the other phenomena; Newton derives gravity from celestial phenomena of the solar system, and then deduces other phenomena of the solar system by the gravity. Certainly Newton thinks that he can solve other natural phenomena by his main idea or principle of philosophy.

In fluid mechanics, we use Euler equation and Navier-Stokes equation to describe the phenomena of the fluid motions; Euler equation and Navier-Stokes equation use pressure forces, friction forces, buoyant forces, and coriolis forces etc.
It is obvious that Euler, Navier and Stokes did not fully understand Newton’s Principia, and did not follow Newton’s main idea or the mechanical principle.
Newton wants to derive forces from phenomena of motions, but Euler, Navier and Stokes do not; Newton wants to deduce other phenomena by the forces, which are derived from the phenomena of motions, but Euler, Navier and Stokes deduce the phenomena of fluid motions by pressure forces, friction forces, coriolis forces, and buoyant forces etc. and none of these forces is derived from phenomena of fluid motions.
Euler, Navier and Stokes omit the first process of Newton’s main idea or the mechanical principle: from the phenomena of motions to investigate the forces of the nature.
Why does Newton emphasis that a force must be derived from the phenomena of motions?