惠子 Huìzǐ, "Master Hui," also known by his personal name, 惠施 Huì Shī, was Zhuangzi's best friend. He is known to have composed a set of laws for King Hui of Wei, who ruled from 370-319 (Lushi Qunqiu, Shěnyīnglǎn Yící 8). This is the best hard evidence we have for Zhuangzi's rough date. The facts that Zhuangzi refers to him as 夫子 fūzǐ, "teacher" or possibly just "sir" (1:06) and that he died before Zhuangzi (9:13) suggest that Huizi may have been older.
Anecdotes about him in the Zhuangzi suggest he argued for a kind of monism by demonstrating the contradictions
that arise from divisions in space and time, concluding that people should
"let love wash over all things and treat Heaven and Earth as a single
body" (CTP 33.07). This suggests that, like others of his generation, for instance, Gaozi in Mengzi (2A2), he was a former Mohist. (See Graham 89: 81 for further evidence of Mohist influence.) If so, exactly why and how he split from the group remains unclear.
Huizi has long been described as representing a purely logical perspective, in contrast to Zhuangzi's openness to intuition. While this may be true, I think there is more to the story than that. Though Zhuangzi does not think that logic has all the answers, he is certainly not averse to using it, especially against itself. Another difference between Zhuangzi and Huizi that may be more telling is Huizi's commitment to political activism, which Zhuangzi did not share (9:10). If arguments in the second chapter like 2:06 are directed at Huizi, the point would appear to be that there is an incompatibility between the end (unit) and the means (logical distinctions), rendering the project futile. Zhuangzi often bemoans his friend's lot and tries to dissuade him from it (5:06).
Lee Yearley suggested that Huizi may have been Zhuangzi's intended audience, his reason for writing.