Regressus
from 1450 Developed at Padua through the fifteenth and sixteenth centuries
Work back from the effect to its cause, then forward again from the cause to the effect — and only the return journey counts as knowledge.
- Active
- from 1450
- Where
- Padua
- Also
- regressus demonstrativus
- Status
- In use
- Tradition
- Classical procedures
What it is
The regressus has two movements. The first, resolution, reasons back from an observed effect to a proposed cause. The second, composition, reasons forward from that cause to the effect, showing that it must produce it.
Between them sits a stage the Paduans argued about for a century: the examination in which the proposed cause is scrutinised until it is known well enough to serve as the starting point of the second movement. Without it the argument is circular, and the whole difficulty of the method is packed into that stage.
How it works
From the effect, reason back to a cause that would produce it. Examine the proposed cause independently until it is understood in its own right. Then reason forward from it to the effect, as a demonstration of why.
The order matters. The first movement discovers, the second proves, and confusing them produces either an unjustified hypothesis or a circle.
The moves, in order
- Observe the effect
- Reason back to a cause that would produce it
- Examine the cause until it is known independently of the effect
- Reason forward from cause to effect
- The forward movement is the demonstration
The procedure
Proposition The tank is emptying because the float valve is passing.
- The effect: the tank drops sixty litres overnight with every outlet closed.
- Resolution: reasoning back, the candidates are a leak in the tank, a leak in the line, or the float valve passing water back down the overflow. Only the third would produce a loss with no wet ground anywhere.
- Examination: the valve is removed and inspected on the bench. Its seat is scored and it does not seal under hand pressure, which is established without reference to the tank at all.
- Composition: reasoning forward, a valve that does not seal will pass at line pressure; at the measured pressure a seat scored to that depth passes roughly forty litres an hour; the overflow discharges to the paddock, where a small continuous flow leaves no visible mark.
Therefore The valve was the cause, and the forward argument shows why that cause produces exactly the loss observed, including the absence of wet ground that the other candidates could not explain.
The bench inspection is the whole method. Without it the argument would be it must be the valve because nothing else fits, which is a hypothesis. With it, the forward demonstration proceeds from something known in its own right.
What it is good for
Diagnosis where the cause can be examined separately from its effect — mechanical, medical, forensic. It is also the best account anyone gave, before the seventeenth century, of how discovery and proof differ.
Where it fails
The middle stage is the weak point and the Paduans knew it. If the cause can only be known through the effect it explains, the examination has nothing independent to work with and the two movements collapse into a circle. Many interesting causes are exactly like this.
It also assumes that effects have single causes clean enough to be isolated. Where several partial causes combine, the resolution stage returns a list and the composition stage cannot be run.
Common misconceptions
That it is simply hypothesis and test. It is close, but the composition stage claims more than testing does — it claims to demonstrate that the cause must produce the effect, not merely that it is consistent with it.
Where it came from
Developed at Padua by Zabarella and his predecessors, working on Aristotle's remarks about the difference between demonstration that and demonstration why.
What became of it
Long argued to have influenced Galileo, who was at Padua while Zabarella taught there, though the strength of that claim is contested. Its substance survives in scientific method rather than its name.
Lasting influence
It is the clearest pre-modern statement of the distinction between the context of discovery and the context of justification, arrived at three centuries before those terms existed.
Why it still matters
Any diagnostic procedure that insists on confirming the cause independently before accepting it is running this.
Put another way
Following footprints to a door, then opening the door to check the boots.
Texts
Jacopo Zabarella, De Methodis
The fullest treatment, including the argument about the intermediate stage that the whole method depends on.
Where to start
Diagnose something yourself and notice whether you confirmed the cause independently or simply stopped when a story fitted.
The surprising bit
A method worked out by Italian professors arguing about a passage in Aristotle turns out to be a fair description of what a good mechanic does.
In their own words
The regress is from effect to cause and back again from cause to effect
Who held it
the Paduan school
Connections
- Parallel to
- Abduction
The verdict
The most sophisticated method of the late scholastics, and still the right shape for diagnosis.