Franciscus Sylvius (Dutch: [frɑnˈsɪskʏs ˈsɪlvijʏs], Ecclesiastical Latin: [franˈtʃiskus ˈsilvi.us]; born Franz de le Boë;[needs IPA] 15 March 1614 – 19 November 1672) was a Dutch physician and scientist (chemist, physiologist and anatomist) who was an early champion of Descartes', Van Helmont's and William Harvey's work and theories.

[3] Sylvius, a Latinization of "de le Boë" translated as "of the woods", was born in Hanau to an affluent family originally from Cambrai, but worked and died in the Netherlands.

[4] In 1634 he held a dissertation titled Positiones variae medicae (Various Medical Positions) under the direction of Vorstius,[5] in which he defended the proposition that there should be a pulmonary circulation.

[5] On 16 March 1637 he defended a doctoral thesis titled De animali motu ejusque laesionibus (On Animal Movement and its Disorders) at the University of Basel under the direction of Emmanuel Stupanus.

Specifically, he explained that digestion is a result of the chemical reactions of acids and bases coming from pancreatic, stomach, and saliva secretions.

He owned a collection of 190 paintings, nine by Frans van Mieris and eleven by Gerard Dou, in the 17th century highly valued and pricey painters.