A doctor, also named Claudius Galenus or Aelius Galenus.

Galen's understanding of anatomy and medicine was principally influenced by the then-current theory of the four humors: black bile, yellow bile, blood, and phlegm, as first advanced by the author of On the Nature of Man in the Hippocratic corpus.[11] Galen's views dominated and influenced Western medical science for more than 1,300 years. His anatomical reports were based mainly on the dissection of Barbary apes.[12] However, when he discovered that their facial expressions were too much like those of humans, he switched to other animals, such as pigs.

Holy shit. So he felt bad about dissecting the apes alive, but didn't stop, just changed the type of animal so he'd feel less sympathy. That's like muffling someone when their screams make you feel guilty.

The reason for using animals to discover the human body was due to the fact that dissections and vivisections on humans were strictly prohibited at the time.[13] Galen would encourage his students to go look at dead gladiators or bodies that washed up in order to get better acquainted with the human body. His anatomical reports remained uncontested until 1543, when printed descriptions and illustrations of human dissections were published in the seminal work De humani corporis fabrica by Andreas Vesalius[14][15] where Galen's physiological theory was accommodated to these new observations.[16] Galen's theory of the physiology of the circulatory system remained unchallenged until ca. 1242, when Ibn al-Nafis published his book Sharh tashrih al-qanun li’ Ibn Sina (Commentary on Anatomy in Avicenna's Canon), in which he reported his discovery of pulmonary circulation.[17]

Galen saw himself as both a physician and a philosopher, as he wrote in his treatise titled That the Best Physician Is Also a Philosopher.[18][19][20] Galen was very interested in the debate between the rationalist and empiricist medical sect, and his use of direct observation, dissection, and vivisection represents a complex middle ground between the extremes of those two viewpoints.

Regarded the natural world as the result of constant mingling and separation of four basic elements.

Whereas Thales said "all is water" and Heraclitus said "all is fire", Empedocles said "all is fire, water, earth and air".

Birthday: 23 March

Developed and popularised Bayesian probability theory (i.e. a way to think about and use probability in real life) – and many other things. Sometimes called the French Newton.

lived 546 BCE

Said the Earth is round (and 300 years later, Aristarchos successfully calculated its circumference). Rejected Thales' idea that the "ur-substance" was water because "everything would have converged into water".

Had an idea of evolution, that humans must have come from fish or some such (correct!). 2200 years later, characters such as Jean-Baptiste Lamarck and Charles Darwin fleshed it out, though they may not have known of Anaximander.

Said to be the first person to draw a map (let that sink in for a moment). I doubt it's literally true, but imagine having invented maps. Next time you open the map on your phone, take a moment to thank Anaximander.

Liked practical research, unlike the later Greeks.

His student Anaximenes, who said the earth was a disk, was liked more during antiquity.