Abu’l-Barakat al-Baghdadi (born c. 1080, Balad, present-day Iraq—died 1164/1165, Baghdad) was an Arab physician, scientist, and philosopher who critically examined the accepted scientific and philosophical ideas of his day and developed intriguing and provocative alternatives. His work in physics is especially notable. He also made important contributions to the understanding of human psychology.

Little is known about al-Baghdadi’s life. Born into a Jewish family, he studied medicine in Baghdad and became a renowned physician, serving in the courts of the caliphs of Baghdad and the sultans of the Seljuq empire. Late in life he converted from Judaism to Islam.

Al-Baghdadi’s magnum opus is the Kitab al-Muʾtabar, whose title has been rendered in English as “The Book of What Has Been Established by Personal Reflection” and “The Book of the Culminating Conclusion.” In the work’s three books—Logic, Physics, and Metaphysics—al-Baghdadi assesses the ideas of Avicenna (980–1037), the most famous scientist and philosopher of the medieval Islamic world, and examines a broad range of scientific, metaphysical, and psychological topics, including motion, time, apprehension, cognition, self-awareness, knowledge (see epistemology), the relation between existence and “existents,” spiritual forms, and God.

As a physicist, al-Baghdadi is best known for his innovative ideas about motion. He rejected the Aristotelian thesis that applying a constant force to an object makes it move at a constant speed and proposed instead that a constant force would cause an object to accelerate. Al-Baghdadi described motion in terms of opposing forces, which he called “inclinations.” He proposed that there are two types of inclinations: “violent” and “natural.” Al-Baghdadi followed Avicenna in proposing that a projectile moves because of a violent inclination produced by the projecting body. In the example of a rock thrown upward, the thrower imparts a violent inclination to the rock. The violent inclination works against the rock’s natural inclination to return to a state of rest. When the rock starts to fall to the ground, the violent inclination weakens as the natural inclination strengthens. The natural inclination continues to build as the rock falls, causing acceleration.

Al-Baghdadi’s notion of acceleration was an early forerunner of Isaac Newton’s second law of motion, typically formulated as F = ma, which states that force (F) is equal to mass (m) multiplied by acceleration (a). Al-Baghdadi also suggested that motion is relative. This idea—that an object’s motion is properly defined as a change in its position relative to an object or place that is not moving—is another fundamental concept of modern physics.