This thinking does not constitute a case of proving or following a proof of the conclusion, because it involves a step which we cannot clearly tell is valid. This is the step of attempting to visually imagine what would happen when the smaller circle varies in diameter between 0 and the diameter of the larger circle and inferring from the resulting experience that the line joining the centres of the circles will always be longer than the horizontal line from the centre of the smaller circle to the vertical diameter of the larger circle. This step seems sound (does not lead us into error) and may be sound; but its soundness is opaque. If in fact it is sound, the whole thinking process is a reliable way of reaching the conclusion; so in the absence of factors that would make it irrational to trust the thinking, it would be a way of discovering the conclusion to be true.