Abduction and diagrams AHTI-VEIKKO PIETARINEN, Nazarbayev University; Research University Higher School of Economics; Tallinn University of Technology. Abstract Abductive conclusions are drawn in a special, co-hortative mood (Peirce’s ‘investigand’). Abductive conclusions are representative interpretants that represent abduction (or retroduction) as a form of reasoning that can convey a general conception of the truth. The truth is not asserted; abduction merely delivers the idea of a matter of course, rendering that idea comparatively simple and natural, hence assuring us of its justified assertibility. Hence abductive reasoning is at home in addressing ‘How Possible’-questions in science. Abductive reasoning concerns the question of how things might, could or would conceivably be such that they can be plausibly asserted. Peirce took all reasoning to be diagrammatic and representable using the graphical method of logic. Yet no examples have previously been found in his large manuscript corpus of what such non-deductive graphs might look like. This paper proposes a new interpretation of a sole exception, a sketch of two graphs from a rejected page from 1903, which might be the only surviving example of Peirce’s abductive graphs. The proposed interpretation takes them to be representative interpretants of this special inverse type of inference. Keywords: abduction, diagrams, Peirce, co-hortative mood, semiotics, economy, uberty 1 Introduction Peirce took abductive conclusions to be drawn in a special, co-hortative mood which he termed ‘the investigand’ (R L 384). 1 Such conclusions are representative interpretants that represent abduction, or what he since 1898 had often termed retroduction, as a form of reasoning that conveys a general conception of the truth [48]. The truth is not asserted; abduction merely delivers the idea of a matter of course, rendering that idea comparatively simple and natural, hence assuring us of its justified assertibility. Hence abductive reasoning is at home in addressing ‘How Possible’ questions 1 This comes from an unsent draft letter to Victoria Welby: This ‘interrogative mood’ does not mean the mere idle entertainment of an idea. It means that it will be wise to go to some expense, dependent upon the advantage that would accrue from knowing that Any Some S is M, provided that expense would render it safe to act on that assumption supposing it to be true. This is the kind of reasoning called reasoning from consequent to antecedent. For it is related to the Modus Tollens thus: Instead of ‘interrogatory’, the mood of the conclusion might more accurately be called ‘investigand’ and be expressed as follows: ‘It is to be inquired whether A is not true.’ The reasoning might be called ‘Reasoning from Surprise to Inquiry’ (R L 463, July 16, 1905, added emphases). The letter soon breaks off, but the coinage of ‘investigand’ is highly suggestive. It does not merely concern forming a weak question (‘Whether A?’, ‘Maybe A?’) as a conclusion of the abduction. The idea of the investigand is that it harbours reasons why this conclusion indeed is pursuit-worthy, in comparison to other, potentially and comparatively like-minded hypotheses. Vol. 29, No. 4, © The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com. Advance Access published 9 April 2020 https://doi.org/10.1093/jigpal/jzz034 Downloaded from https://academic.oup.com/jigpal/article/29/4/447/5734760 by guest on 01 June 2022