I am looking for the source and context of this quote, found e.g. at St Andrews:
Only with the greatest difficulty is one able to follow the writings of any author preceding Euler, because it was not yet known how to let the formulas speak for themselves. This art Euler was the first to teach.
— F. Rudio
(My emphasis. Slight variants suggest that it could originally have been in another language.)
The quote is from a speech Rudio gave at the Town Hall in Zürich on the 6th December 1883; The German original is published in Felix Stähelin, Reden und Vorträge (1956, I have not found it online).
An English translation of the full speech is here. The translated quote reads as follows:
But I cannot move on from reviewing Euler's mathematical work without having considered an important factor. I have said that mathematics is a language in which natural phenomena can be described in the simplest and most comprehensive manner. With this in mind, you will understand how important it is to express mathematical thoughts themselves as concisely and clearly as possible. In this respect, Euler's work was epoch-making. We can be safe to say that the whole form of modern mathematical thinking has been created by Euler. If you read any author immediately before Euler, it is very difficult indeed to understand his terminology, as he has not yet learned how to let the formulas speak for themselves. This art was not taught until Euler came along.
