" Moreover, that two methods –demonstrative and resolutive– are sufficient for knowing all things can easily be shown. For everything set out to be known is either substance or accident.
A substance, of course, is fully known whenever its perfect definition is had. If this is known, then no method is needed for it to be investigated. And if it is unknown, it has to be searched for by means of some method. We cannot, of course, search for it by means of demonstration, as Aristotle says in text no. 42 of the second book of the Posterior Analytics. For only those things whose essence depends on some external cause can be made known a priori and by means of the cause. But the essence of a substance depends on no external cause. There is no cause, therefore, by means of which the definition of a substance, if it was unknown, could be demonstrated. It remains that it cannot be made clear unless from posterior things and from some effect. This is resolutive method.
But now accidents: some are proper, others common. The common, of course, does not fall under science, but the proper always has some definite external cause on which it depends and by means of which it can accordingly be demonstrated. I say external, not because it [the cause] is always separated in place and subject [from the accident], but by reason of the essence of the accident itself; it is necessary that it [the cause] is outside this [outside the essence of the accident]. But whatever has such a cause, can in no way be known scientifically unless it is demonstrated by means of that [cause]. Every accident, therefore, is demonstrable and there is no way except demonstrative method that could lead to perfect scientific knowledge of it.
And so, since we know all substances by a definition and we investigate their unknown definitions by means of resolution alone, and accidents become known by means of demonstration alone, these two methods are sufficient for procuring knowledge of all things. Nor are we in need of any other logical instrument for proving and gathering the unknown from the known. Of course, we need [such] for disposing, because method does not dispose, but it proves and makes known one by one those things that have been disposed using appropriate order. And so neither order alone nor method alone can provide perfect scientific knowledge of things; instead both are required. For order alone teaches nothing, and method without order teaches, of course, but does not bring forth scientific knowledge or [does so] with the greatest effort in learning and with the greatest difficulty.
...But if every thing being treated has been disposed using appropriate order, method really does its work very well in making each of the things clear and brings forth perfect scientific knowledge in which the soul of learners rests. Accordingly, method sometimes also needs division, because division is a sort of ordering that furnishes to methods the correct sequence of the things that are to be proved.
From the very nature of the things to be known, therefore, it is clear that those two methods are sufficient for conveying knowledge of all things.
The same is shown from the very progression of method, as we said earlier. For every scientific progression from known to unknown is either from cause to effect or from effect to cause.
The former, of course, is demonstrative method, the latter resolutive.
There is no other procedure that brings forth certain knowledge of something. For if we progress from something to something, of which neither is the cause of the other, there cannot be an essential and necessary connection between them, and so no certain knowledge can ensue from that progression. It is patent, therefore, that there is no scientific method besides demonstrative and resolutive. "
Jacopo Zabarella | On Methods