From an unclean thing what will be made clean? And from something false what will be true?: No answer is expected to these questions; they are only for effect. The implied answer to both is “Nothing.” Good News Translation combines the two questions into one statement. Since dreams are not real (they are false), they cannot give rise to anything that is real. They are compared to unclean things here. Ben Sira probably has in mind ritual uncleanness rather than physical uncleanness, but in this context the distinction is not important; consequently Good News Translation “dirty” serves the purpose. Compare Job 14.4. Translators who are using poetry can keep the two-line structure here if they wish by using the following model that is similar to Contemporary English Version:

• Something clean cannot come from something dirty,
and what is true cannot come from something false.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Sirach. (UBS Helps for Translators). New York: UBS, 2008. For this and other handbooks for translators see here.