This verse is very similar to Verse 4 in Chapter 1, and thus, has a similar explanation. If there were six doubles and not seven, then we might conclude that the support of the Holy Palace was unnecessary and that the universe is purely mechanical with no need for God. If we said there were eight and not seven, then this would add another vertical path and another sepfirah to the Tree of Life. From this we might erroneously conclude that God is a sefirah that can be defined and contained within the forms created by our own consciousness.
A subtle difference between this passage and the parallel one in Chapter 1 is that here it says "probe with them" instead of "probe from them." The double letters represent the very tools with which we are to examine the universe and grow. In a more general sense, there are paths along which we have the ability to grow and develop, and we are obligated to persue these paths. It is an obligation to increase our wisdom, and it is an obligation to develop peace and wholeness. If we do not leave this life more evolved than we were when we came into it, then we have wasted a precious opportunity.
The last sentence in this verse reiterates that we must see the true spiritual essence of everything we encounter and be aware of the unchangeable Creator underlying all creation.