把“知之为知之,不知为不知,是知也”翻译成英语,

中文意思是：知道就是知道,不知道就是不知道,这才是聪明智慧的表现.
最后一个“知”是“智”的通假字
所以英语翻译过来应该是
It is wise to hold what you know and admit what you don't know.
我觉得这样能够比较简短
网上还有比较不错的说法
When you know a thing,to hold that you know it; and when you do not know a thing,to allow that you do not know it;-this is knowledge.
Hold what you really know and tell what you do not know -this will lead to knowledge.