Spinoza
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Born just a month after Leeuwenhoek, Spinoza ended up grinding lens a few miles away. He lived in Voorburg, four miles up the Vliet from Delft, from 1663-1670. He spent the final seven years of his life in the Hague, just a few miles farther up the canal.
The two had friends in common, most prominently Christiaan Huygens. Oldenburg visited Spinoza (but not Leeuwenhoek) and corresponded with them both. Leibniz visited them both during his trip to the Dutch Republic in 1676. They clearly knew of each other, though there is no evidence they ever met or corresponded.