From his boyhood he was encouraged by the mathematician Louis-Philippe Gilbert but at first Vallée Poussin thought he would become a Jesuit priest.
Mula sa kanyang kabataan siya ay hinihikayat sa pamamagitan ng mga dalub-agbilang Louis-Philippe Gilbert ngunit sa unang Vallee Poussin-iisip siya ay maging isang Heswita pari.