The papers by Isaac Levi and by Dudley Shapere, despite their many differences, share a common theme. This is the idea that science in a certain sense builds upon itself, that some of its assertions, having become firmly established, play important roles in the further development of science. Established science not only guides practical action, it also constrains possible conjectures, guides the construction of more extended and related theories, and even guides the construction of their eventual successors (though this last needs very careful handling). This basic idea is surely correct; but I have many disagreements with the particular (and different) ways that the basic idea is developed by Isaac Levi and Dudley Shapere. I shall therefore first, outline very roughly the sort of development of this basic idea that I would advocate.