The Doctor is confusing an angel to death.
Light came to our world to count and quantify all life, to create a set and definitive catalogue. Light sent its Survey out into the world to sample each form. But our world corrupted the Survey with the delicious possibilities of evolution. Light was locked away so the Survey could inherit the Earth. It became a Victorian gentleman, a man of property. It enacted a ruthless Darwinian takeover of the house above Light's ship. A colonizing mission. A merger and acquisition. This being Victorian England, the wife and daughter and maids came with the house like fixtures and fitting. The Survey locked its secrets away, just like any Victorian gentleman, and set about dreaming of empire. It adopted the cultural logic of its new society and new position: the ideology of 'the survival of the fittest'... meaning, supposedly, the dominance of the best. With its inbuilt assumptions about the place of 'lesser races' and 'lower orders' and women, Victorian social-Darwinism was perfect for the Survey's purposes, as it shed its insectile and reptilian skins and became Josiah, the pink of respectability.
But then the Doctor came and let Light out of the cellar. Just to see what would happen.
Light turned out to be the reductionist ghost in the bourgeois social machine.
To Light, we're merely walking bowls of "sugars, proteins and amino acids". Light killed and dismembered one of the maids, saying "I wanted to see how it worked, so I dismantled it". That's just how reductionism works. To understand something, you take it to pieces. But what happens when you can't put the pieces back together again? Do you forget that the original thing was more than just the sum of its bits? Reductionism can do a lot of heavy lifting as an analytical tool, but it is the map not the territory... and mistaking it for the territory leads to vulgar materialism and determinism. A river cannot be understood as just an aggregation of water molecules. Aside from all the other natural and material processes involved, it is also a social phenomenon. It is something people experience, think about, wade in, swim in, sail upon, fish for food in, divert and ford and dam and befoul. It is something people name, and build towns around. Water can be used to quench thirst or drown people. It can be freely shared or owned and monopolised, or stolen. Likewise - more so - people are not just aggregations of limbs or genes (selfish or not). Looking at them that way makes it possible to inherit them and use them like property. Contrary to the assumptions of bourgeois political economy, societies are not just aggregations of individuals, all acting from their own self-interest. That's part of how you end up saying that some people just have to be left to starve, or be put in the workhouse, or be ruled by a Viceroy, all for the good of the economy and progress. It's partly how you end up with the idea that people starve or work or serve because they have failed to compete, or because it was their destiny as a unit of inherently inferior stock. Inspector Mackenzie has imbibed this view of things, sagely pronouncing on how "gypsy blood" makes for "lazy workers".
This view of the world depends upon snapshots of reality at best, all fixed in place like moths displayed behind glass, like catalogued specimens in the Natural History Museum. There is something about this static view that makes it tesselate perfectly with hierarchy, and thus work for whoever rules. That's why the classic depiction shows a lowly ape gradually growing up to be a white gentleman. It depends upon forgetting the revolutionary implications of Natural Selection, which shows us a world of variation in dialectical unity, everything effecting everything else in one great network of feedback loops, all species constantly on their way to being something else, all forms transitional, all races related to each other, no hierarchy of blood, no separation of individuals from each other, no dividing line between individuals and the rest of the world, every tiny alteration in quantity gradually leading to an alteration in quality, all negations ultimated negated, everything containing its own contradictions within it... just as every apple contains the potential to nourish or rot.
In truth, as Light realises to his horror...
"Everything is changing. All in flux. Nothing remains the same."
The mercurial Doctor has reminded Light that even he, Light, changes. Everything does. The catalogue can never be complete, by definition. The Doctor cruelly hammers home the word "change" at every opportunity. He bamboozles Light with a list of mythical and fictional creatures, human creations, inherently social things that can never be quantified as part of any static, reductionist system. Even the Gryphon gets a mention, that creature of Victorian lassitude and melancholy, yearning for the old days before everything changed.
Even Nimrod, whose people once worshipped Light, won't help him. Nimrod has dumped his allegiance to both Josiah and Light, both the new boss and the old. He can't be fixed in subservient place because he's a social creature who thinks and learns and makes his own history, if not in circumstances of his own choosing.
"I will not change," says Light. And he turns to stone rather than permit himself to become part of the great flow of fluctuation, contradiction and transformation. He's that reactionary.
"Subject for catalogue," announces the Doctor with weary contempt, "File under: Imagination, comma, lack of."