If you accept the basic notion that the Voynich Manuscript is both (a) very probably a genuine (if perhaps rather convoluted) cipher, and (b) mostly rational, then you run into the issue of what kind of sensible stuff lies beneath – in other words, its “secrets”. All the same, how sure are we that our modern notion of “secrets” is anything like the Early Modern / Renaissance notion of “secrets”? However tempting it may be, back-projecting what we think and know now onto what people thought and knew several centuries ago is very often significantly wrong – in fact, this is one of the major sources of historical errors.
For example, I would argue that the original meaning of “secrets” has become progressively diluted by a centuries-long barrage of religious and political propaganda dressed up as conspiratorial claims. Perhaps the best-known example of this is the polemical vitriol directed against the Jesuits from around 1600 onwards (less than a century after the Society of Jesus was formed), and which even now finds expression in contemporary works – Dan Brown’s fumbling portrayal of Opus Dei machinations in The Da Vinci Code is essentially 17th century anti-Jesuit propaganda dressed up in 20th century garb.
Going back further still, Carlo Ginzburg attracted both bouquets and brickbats (in roughly equal measure) by suggesting that the stories told about witches by the Inquisition bore many striking resemblances to the stories told in previous centuries about Jews (poisoning wells, etc). None of these stories had any basis in reality – yet ultimately they are the sources that people rely upon when they talk about the suppression of heresies.
Somewhere along the line people progresssively forgot that this was just political propaganda, and the notion of the ‘Big Heretical Secret That Must Be Hidden By Any Means’ started to assume centre stage. I defy anyone to point me to any Big Heretical Secret that was in any way cryptographically concealed. (Note that (a) the jury is out on the Rohonc Codex; (b) if the Turin Shroud does turn out to be a genuine artefact brought back from the East by the Knights Templar, it would be ultra-orthodox rather than ultra-heretical; and (c) don’t even think about raising the so-called Bible Code).
(Of course, this is the point where some like to counter that the Big Heresy that was concealed must be So Very Big that we’re automatically blinded to it by our politico-historico-religious acculturation. To which I reply: even though my eyes are wide open, I continue to see nothing even remotely close.)
In actuality, every single Early Modern secret I’ve come across to date is simply what we would nowadays call a “trade secret”. Whether the trade is respectable (paint-maker, apothecary, glassworker, optician, architect, engineer, metalworker) or not (alchemist, necromancer, perpetual motion maker, perpetual light maker, empiric, politician 🙂 ), what they wanted to keep secret was “how to” procedural knowledge.
Roger Bacon’s statement that one should “not cast pearls before swine; for he lessens the majesty of nature who publishes broadcast her mysteries” stands firmly on the rock of esotericistic mystification implicit in the well-known “Secretum Secretorum“: but in my opinion this primarily referred to veiling the (supposed) secrets of natural science from those living outside Academe’s leafy vale. And in the end, this particular bubbling tureen of fringe knowledge reduces down to a small bowlful of alchemists’ trade secrets.
The whole “Secret History of Secrets“, then, comes down to one thing: rather than being heretical, they were useful – that is, not ideas to change or topple religious worldviews, but ways to help people do things.
In this general vein, here are a couple of nice Renaissance trade secrets I’ve noted recently. Firstly, a report that some Venetian paintmakers or painters seem to have added finely ground glass to their paints, presumably to try to produce a luminous effect; and secondly, that Antonello da Messina may have been the missing link / roving master that brought oil painting secrets from Van Eyck to Venice (where all the other painters got it from). Incidentally, Giorgio Vasari alleged that Van Eyck was both a painter and an alchemist (which I didn’t know): and in fact there is a whole mad literature hunting for alchemical symbolism in Van Eyck’s work… not my kind of thing, sorry.