Richard Hanson and Coryn Bailer-Jones (both MPIA) and I met today to talk about spatial priors and extinction modeling for Gaia. I showed them what I have on spatial priors, and we talked about the differences between using extinction measurements to predict new extinctions, using extinction measurements to predict dust densities, and so on. A key difference between the way I am thinking about it and the way Hanson and Bailer-Jones are thinking about it is that I don't want to instantiate the dust density (latent parameters) unless I have to. I would rather use the magic of the Gaussian Process to marginalize it out. We developed a set of issues for the document that I am writing on the subject. At Galaxy Coffee, Girish Kulkarni (MPIA) gave a great talk about the physics of the intergalactic medium and observational constraints from the absorption lines in quasar spectra.