When I see a tomato there is much that I can doubt. I can doubt whether it is a tomato I am seeing or a cleverly painted piece of wax. I can doubt whether there is any material thing there at all. Perhaps what I took for a tomato was really a reflection; perhaps I am even the victim of a hallucination. One thing however I cannot doubt: that there exists a red patch of a round and somewhat bulgy shape, standing out from a background of other color patches, and having a certain visual depth, and that this whole field of color is directly present to my consciousness.

If it made sense to say that time flows then it would make sense to ask how fast it flows, which doesn’t seem to be a sensible question. Some people reply that time flows at one second per second, but even if we could live with the lack of other possibilities, this answer misses the more basic aspect of the objection. A rate of seconds per second is not a rate at all in physical terms. It is a dimensionless quantity, rather than a rate of any sort.