Where Vegio saw the basilica of St. Peter’s, begun by the Emperor Constantine in the fourth century, is now the new church commissioned by Pope Julius II from Bramante in 1505 and consecrated in 1626. Since what our author saw and experienced no longer exists, how could we evaluate and interpret his description of it? We needed to know what he included and omitted from his necessarily – and voluntarily – selective account to understand his criteria of choice and hierarchy of values and to share his general impression of the basilica. This knowledge illuminates the purpose and meaning of his text, as we saw in Chapter 3. In the earlier stages of our research, it seemed possible to compare the text to a mental image based on art historical knowledge of the basilica, available plans, and reconstructed elevations. Eventually, though, it became clear that the text is so rich in detail and so complex – partly because Vegio describes not only what he actually saw but also things that had once been, but no longer were, visible – that its variety and richness exceeded our imaginative capacity. Another approach to imaging what Vegio saw is to make a 3D digital reconstruction of the building as it was in the 1450s and compare it to Vegio’s text: This is what we did (Frontispiece and Figure 1).1 We know that any reconstruction of the visual impression of Old St. Peter’s is an act of the imagination and that the result can only be a fiction. Yet it is a fiction that tells us things that are true, so long as the reader shares the reasoning and knows the evidence behind the multiple decisions that determined its appearance.