"Ah, sir: we have someone in Vienna who will crush us all; he is a universal genius compared to whom I am a child."It would be nice to have the original German (I assume the remark was made in German as Haydn's English was not very fluent). But of course, Haydn said something similar to Mozart's father Leopold to the effect that they would not see his son's like again for a hundred years. And that is probably an understatement.
Haydn, despite rising to the height of being the most famous composer in all of Europe, was actually a humble man and had no reticence in praising his dear friend Mozart. Mozart in his turn learned a great deal from Haydn--just compare his string quartets written before he heard Haydn's op 20 and 33 with those he wrote after, and dedicated to the master.
But Mozart was indeed a universal genius and we will likely never see his like again. He was a child prodigy as a composer, not just a performer. We are inundated these days with 13, 12, even 9 year old child sopranos with startling voices. But we have never been inundated with child composers. Apart from a couple of minor examples, we have only Mozart.
I think I have mentioned before playing a little duet with a student many years ago. It was a minuet by Mozart transcribed for two guitars and the very thorough German publisher had indicated the year of composition on the score. After we got to the end I glanced at the date, did a quick mental calculation and said to my young, 9 year old student: "good heavens, do you realize that Mozart wrote this when he was seven years old!" It keeps happening. I am teaching myself piano right now and just was playing another little piece written by Mozart when he was six!
Just to summarize, Mozart began composing when he was five, wrote his first symphony at age eight, his first opera at eleven, his first commissioned opera at fourteen and so on. No, as a matter of fact I know of no others with this kind of precociousness. It was only possible because he was, in the words of Allan Pettersson, "born under a piano". Born, that is to say, into the family of a famous violinist and composer, his father Leopold Mozart. This also created difficulties for Mozart his whole life in terms of separating himself from the excessive influence of his father.
But Haydn, in his enthusiasm, was actually over-stating the case a bit. Mozart is not actually a better composer than Haydn. Don't take my word for it, go and listen to all the symphonies of Mozart and all the symphonies of Haydn. It should take about a month. I'll wait.
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All done? You see what I mean? Mozart wrote several spectacular symphonies, but, like his string quartets, most of his early ones are rather crude and forgettable. And, of course, he wouldn't likely have written any truly great symphonies if he hadn't learned so much from Haydn. Now Haydn, on the other hand, had to figure it all out for himself. He didn't have much in the way of influences. He never knew the music of Bach very well (Mozart only came across it later in life) and the Classical style that we are so familiar with was largely the invention of Mr. Haydn. Who proceeded to write, not several, but one hundred and six symphonies, most of them spectacular.
It is rather ironic that someone once wrote to Beethoven, on the eve of his moving to Vienna, that he would "receive the spirit of (the recently deceased) Mozart from the hands of Haydn". The reality is that Beethoven disliked his lessons with Haydn and instead emulated the forms and structures of a number of pieces of Mozart directly. The greater truth is that Beethoven actually received from Mozart the spirit of Haydn.
Let me see if I can find an example. Here is the scherzo from the Symphony No. 3 of Beethoven:
Now compare it to the last movement of Haydn's Symphony No. 41, chosen pretty much at random (the last movement starts around the 15:12 mark):
The Haydn symphony was written in 1769 when Mozart was thirteen years old and Beethoven wasn't even born yet!
I guess my point is simply that of these three great Viennese classicists, Haydn and Mozart had the greatest mutual respect, while Beethoven admired Mozart and begrudged Haydn's fame. In terms of aesthetic quality, they are all three remarkable, but while Mozart is hugely admired along with Beethoven, Haydn gets much less recognition. He didn't even make the New York Times list of the top ten greatest composers. But he is in absolutely no sense a lesser composer than Mozart or Beethoven. Compared to Mozart, he is also a universal genius.