"The ability to learn from their mistakes."
In the text, Leeuwenhoek stated that he wrote this letter in reply to unnamed "gentlemen" who wanted to know why the wood in some ships rotted more quickly. Did it depend on when the tree was cut? On where the tree had grown?
Leeuwenhoek was accustomed to contradicting and disagreeing with people because of what he had observed. Repeatedly in this letter, he referred to the gemene man (common man). If Leeuwenhoek didn't know enough himself, he turned to experts.
As regards the Wood that is hewn in Winter or in Summer, of this, the common saying in our Country is: Wood that is hewn in the Winter is close, and much firmer than Wood that is hewn in the Summer. But as this was contrary to my opinion, I have approached several Persons whom I judged to have the best knowledge of Wood; that is, those who were either continually working with it or, maybe, hewing it. Among several I found only one who told me that he had many time experienced ...
Leeuwenhoek had some empathy for the common man's erroneous views:
If we look into the reasons which the common man appears to hold, why the Wood is looser and more brittle in Summer than in winter, we cannot take it amiss of him.
And again, referring to "some":
Some try to explain to us that, when we cut into an Elm-tree when it stands and grows, this causes worms to come into the tree. But I assert that the cause is only this ...
And again, referring to "the general opinion":
Now in order to distinguish the very firmest, strongest and most durable Wood from the loosest, weakest and least durable Wood, the general opinion (as my own was a few years ago) is that Wood which grows in a good soil, and very slowly, is the strongest, firmest and most durable Wood; and however many Carpenters I asked about this, they all testified (not knowing any better) that the Wood which grew thick in a very few years is very brittle wood, and therefore easy to work up. But when I had put my ideas about this to some few of the Carpenters who, in my judgement, had the ability to learn from their mistakes, they were amazed, and had to admit to me that nearly all Wood which increases rapidly in thickness is the firmest, most durable, and strongest Wood; and I convinced them still more of this when they had bought an Elm-tree that had grown in our City, and of which it was known that it had stood about 80 years, and when I pointed out to them how one could see at the tree how old it was.
Toward the end of the letter, he wrote:
Because many cannot understand how barrels made of oak can contain liquids, since the wood consists of no other parts than of the tubules that are joined together, I cannot refrain from pointing out how, and in what manner, oak is worked from which Barrels and Casks are made.
By that time, 1686, Leeuwenhoek had been Delft's wine gauger for many years, so he was familiar with the variety of barrels that held liquids.
And again, referring to "many persons":
Many persons might argue, as I have said so much about the soundness of the oak that grows in warm Countries, what might be the reasons why Ships - and especially ships of War - that are built in Denmark and Sweden from Wood that grows in those Countries, can remain good for so long; but let such persons reflect that...
At the end of the letter, he wrote:
A few weeks ago I was watching a Mast of a large ship being got ready. On seeing this Mast at its end, I said to the Workmaster: that Wood did not grow in Norway, it is frail Wood. And pointing to part of another Mast, I said: that Wood is still more unsound. This my judgement I based only on the fact that the rings of this Wood were far apart, and that, therefore, the tree had fast increased in size, since each circle is one year's growth.
To this I received the reply that the Masts had come from Hamburg.