Paul-Yves Pezron (1639, Hennebont, – 9 October 1706, Brie) was a seventeenth-century Cistercian brother from Brittany, best known for his 1703 publication of a study on the common origin of the Bretons and the Welsh, Antiquité de la nation, et de langue des celtes. Pezron was a Doctor of Theology at the Cistercian College of St. Bernard in Paris and abbot of La Charmoie.
In his time, he was known in France as a chronologist. Pezron traced Welsh and Breton origins to the Celts of ancient writers, and traced the Celts further to eponymous hero-patriarchs from Gaul to Galatia. Pezron believed the Welsh language came from a mother tongue called Celtick, a language that was only a theory to other authors. Pezron's fairly unscientific book was popular and reprinted until the early nineteenth century.
The book by Pezron was said to be somewhat unscientific. This claim seems a little unfair. I have read the English print version of the 18th century and there are some impressive references. Perhaps they are doubted, but they are copied from their original languages. As well, there is quite the list of words from other languages that Pezron claims had Celtic origin. They seem quite believable to me, or at least offer significant possibility. They are evidence and not to be readily dismissed without counter-evidence. Certain, the author would not be overstated in hoping for more research into the claims of Pezron.
The biggest problem is that the time period referred to, as the expansion of the sons of Gomer, predecessor of Celts and Goths, is so remote that there is no evidence to say, one way or the other. I estimate the time period to be anywhere from 1700 BCE to perhaps 2100 BCE. Aside from some inscriptions, we have little from that time.
In fact, the evidence Pezron does give us, is perhaps the most and best evidence we have available, whether enough or not. Without counter-evidence, it should not be heavily disputed. Pezron also refers to other writers whose works are still in existence, such as Tertullian. To me, unscientific means it was submitted without evidence. This would be incorrect. It had evidence. Whether that evidence is enough to satisfy all or some, would have to be left to readers of Pezron's work.