It's not so much a definitive statement as a kind of inclusion. Thinking with my hands does not mean it's done without my head. The more I hold stones (large or small) I just find it hard to think without using my hands as well. To not hold them is to lose an important cognitive factor. Consciousness of the potential within stone and the awareness of what is waiting to be done with, or understood about 'stones', is just made more possible through holding them and playing with them. These are after all, the precursor to LEGO, and the forerunners of clay bricks, cement blocks, steel beams, all sorts plastic building material and various substances being used now in 3D printing processes. Each new non-stone material ends up being less handled than the previous, and leaves us more and more removed from having a creative one-on-one experience .
Stones have been around along time before men invented language, before the advent of books, television, computers. They helped us learn to think before we learned why we needed to think, to create before we knew why. We still don't know why, but Im guessing that our collective hands-on experience with stones over the eons of primitive generations surpasses any other knowledge we have attained, even if it's only carried subconsciously in our genetic makeup. It does our head good once in a while to recognize and respect (and look for) the wisdom that comes directly through our hands, by way of stones.
