He was the oldest son of the Swiss-born banker turned theatre principal Abel Seyler and his first wife Sophie Elisabeth Andreae, and was a maternal grandson of the wealthy Hanover court pharmacist Leopold Andreae (1686–1730).
After the death of their mother in 1764, he and his siblings grew up in Hanover with their uncle, the natural scientist Johann Gerhard Reinhard Andreae.
After the death of their uncle, he and his siblings inherited the Andreae & Co. pharmacy, and they appointed Philipp Friedrich David Murray (1770–1828) as its administrator.
[1] On 24 October 1776 he joined the masonic lodge Zum schwarzen Bär in Hanover, and became Worshipful Master in 1778.
His religious name was "Petrus Abaelardus," in honour of the medieval French scholastic philosopher Peter Abelard.