“平衡态”和“偏离平衡”是古人用直觉就把握的世界理论。人们从这一观点出发，逐渐建立一个differential theory（newton laws）。

Aristotelian (384-322 BC): Violent motion (Need persistent causing by an agent) / Natural Motion. Eg. object naturally falls down and stay at rest. Eg. Projectile motion sustained by some intermediate carrier of motion, continuously exercising force to it. Denies empty space. Potentiality and Actuality.

Hipparchus (2CE): force is "transferred" to the body and dissipates as the body moves.

Philoponus (6CE): "inclination" is gained from the agent and then dissipated. Upon exhaustion, the object is back to natural motion. Objects continue to move in empty space. Concept of energy: dynamis, energeia.

(Iran) Avicenna (11CE) The Book of Healing. Distinction between Force and Inclination (Mayl). Object gains Mayl when it is in opposition to its natural motion. (close to potential energy). Similar idea as Newton's concept of Inertia. Dissipation of Mayl requires external force.

(Arabic) Hilbat Allah Abu'l-Barakat al-Baghdaadi (12CE). Like Philoponus' idea, unlike Sina: Mayl self-extinguishes.

Acceleration of falling body: The falling body itself provides mayl, one after another.

Jean Buridan (14CE), impetus = weight x velocity. Similar to today's concept of momentum. When something moves an object by violence, it provides 1) force - that makes the object move, 2) impetus - that keeps exerting force to the object. Still distinguishes moving and at rest. Distinguishes linear and circular impetus.

Posits that God provides celestial bodies with impetuses, that are never damped or resisted. Could not well explain why these bodies are rotating at a constant speed, rather than infinitely fast.

Tunnel Experiment distinguishes between 1) Aristotelian theory 2) H-P Variant of Aristotelian theory, 3) Buridan impetus theory.

1) Predicts that the object becomes at rest when reaches the center of the Earth, since no force would be there to move it. 2) Cannot reach the surface again, since by the time the ball reaches center of the Earth, the initial upward force of impetus is either exhausted, or produce a force opposite to the direction of motion of the ball. 3) Predicts that the object would perform pendulum motion. During the natural motion downward, the object would accumulate enough impetus, greater than gravity, to pull it up toward the surface after reaching the center.

Pendulum motion is not explainable by 1) or 2), only explainable by 3).

This thought experiment is the origin of all oscillations in the history of dynamics. Analogy is quickly drawn between pendula and vibrating strings.

epistemic dynamics / belief dynamics.

c.f. Borel-Cantelli.