Numerous recent publications on the psychological meaning of “if” have proposed a probabilistic interpretation of conditional sentences. According to the proponents of probabilistic approaches, sentences like “If the weather is nice, I will be at the beach tomorrow” (or “If p, then q” in the abstract version) express a high probability of the consequent (being at the beach), given the antecedent (nice weather). When people evaluate conditional sentences, they assumingly do so by deriving the conditional probability P(q|p) using a procedure called the Ramsey test. This is a contradicting view to the hitherto dominant Mental Model Theory (MMT, Johnson-Laird, 1983), that proposes conditional sentences refer to possibilities in the world that are represented in form of mental models. Whereas probabilistic approaches gained a lot of momentum in explaining the interpretation of conditionals, there is still no conclusive probabilistic account of conditional reasoning. This thesis investigates the potential of a comprehensive probabilistic account on conditionals that covers the interpretation of conditionals as well as conclusion drawn from these conditionals when used as a premise in an inference task. The first empirical chapter of this thesis, Chapter 2, implements a further investigation of the interpretation of conditionals. A plain version of the Ramsey test as proposed by Evans and Over (2004) was tested against a similarity sensitive version of the Ramsey test (Oberauer, 2006) in two experiments using variants of the probabilistic truth table task (Experiments 2.1 and 2.2). When it comes to decide whether an instance is relevant for the evaluation of a conditional, similarity seems to play a minor role. Once the decision about relevance is made, believability judgments of the conditional seem to be unaffected by the similarity manipulation and judgments are based on frequency of instances, in the way predicted by the plain Ramsey test. In Chapter 3 contradicting predictions of the probabilistic approaches on conditional reasoning of Verschueren et al (2005), Evans and Over (2004) and Oaksford & Chater (2001) are tested against each other. Results from the probabilistic truth table task modified for inference tasks supports the account of Oaksford and Chater (Experiment 3.1). A learning version of the task and a design with every day conditionals yielded results unpredicted by any of the theories (Experiments 3.2-3.4). Based on these results, a new probabilistic 2-stage model of conditional reasoning is proposed. To preclude claims that the use of the probabilistic truth table task (or variants thereof) favors judgments reflecting conditional probabilities, Chapter 4 combines methodologies used by proponents of the MMT with the probabilistic truth table task. In three Experiments (4.1 -4.3) it could be shown for believability judgments of the conditional and inferences drawn from it, that causal information about counterexamples only prevails, when no frequencies of exceptional cases are present. Experiment 4.4 extends these findings to every day conditionals. A probabilistic estimation process based on frequency information is used to explain results on all tasks. The findings confirm with a probabilistic approach on conditionals and moreover constitute an explanatory challenge for the MMT. In conclusion of all the evidence gathered in this dissertation it seems justified to draw the picture of a comprehensive probabilistic view on conditionals quite optimistically. Probability estimates not only explain the believability people assign to a conditional sentence, they also explain to what extend people are willing to draw conclusions from those sentences. / Zahlreiche aktuelle Publikationen über die psychologische Bedeutung der Worte „Wenn - dann“ schlagen eine probabilistische Interpretation von Konditionalen vor. Vertretern dieses probabilistischen Ansatz zufolge drücken Sätze der Form „Wenn das Wetter schön ist, dann bin ich morgen am Strand“ (oder „Wenn p, dann q“ in der abstrakten Version) eine hohe Wahrscheinlichkeit des Konsequenten (am Strand sein), gegeben den Antezedenten (schönes Wetter) aus. Menschen beurteilen demnach Konditionalsätze, indem sie die bedingte Wahrscheinlichkeit P(q|p) mit Hilfe eines Ramsey-Tests abschätzen (Evans & Over, 2004). Diese Sichtweise stellt einen Gegenentwurf zur bisher dominanten Theorie mentaler Modelle (Johnson-Laird, 1983) dar, die davon ausgeht, dass Konditionalsätze Aussagen über Möglichkeiten machen, die in Form mentaler Modelle repräsentiert werden. Obwohl probabilistische Ansätze in den letzten Jahren überzeugende Evidenz für eine probabilistische Interpretation von Konditionalen präsentiert haben, gibt es noch keine überzeugende probabilistische Erklärung für konditionales Schließen. Die vorliegende Doktorarbeit leistet einen Beitrag zu einer umfassenden probabilistischen Theorie von Konditionalen, die die Interpretation und die Ziehung von Schlüssen aus Konditionalsätzen umfasst.

This thesis is essentially a portfolio of four disjoint yet thematically related articles that deal with some semantic aspect or another of natural language conditionals. The thesis opens with a brief introductory chapter that offers a short yet opinionated historical overview and a theoretical background of several important semantic issues of conditionals. The second chapter then deals with the issue of truth values and conditions of indicative conditionals. So-called Gibbard Phenomenon cases have been used to argue that indicative conditionals construed in terms of the Ramsey Test cannot have truth values. Since that conclusion is somewhat incredible, several alternative options are explored. Finally, a contextualised revision of the Ramsey Test is offered which successfully avoids the threats of the Gibbard Phenomenon. The third chapter deals with the question of where to draw the so-called indicative/ subjunctive line. Natural language conditionals are commonly believed to be of two semantically distinct types: indicative and subjunctive. Although this distinction is central to many semantic analyses of natural conditionals, there seems to be no consensus on the details of its nature. While trying to uncover the grounds for the distinction, we will argue our way through several plausible proposals found in the literature. Upon discovering that none of these proposals seem entirely suited, we will reconsider our position and make several helpful observations into the nature of conditional sentences. And finally, in light of our observations, we shall propose and argue for plausible grounds for the indicative/subjunctive distinction.distinction. The fourth chapter offers semantics for modal and amodal natural language conditionals based on the distinction proposed in the previous chapter. First, the nature of modal and amodal suppositions will be explored. Armed with an analysis of modal and amodal suppositions, the corresponding conditionals will be examined further. Consequently, the syntax of conditionals in English will be uncovered for the purpose of providing input for our semantics. And finally, compositional semantics in generative grammar will be offered for modal and amodal conditionals. The fifth and final chapter defends Modus Ponens from alleged counterexamples. In particular, the chapter offers a solution to McGee’s infamous counterexamples. First, several solutions offered to the counterexamples hitherto are all argued to be inadequate. After a couple of observations on the counterexamples’ nature, a solution is offered and demonstrated. the solution suggests that the semantics of embedded natural language conditionals is more sophisticated than their surface syntax indicates. The heart of the solution is a translation function from the surface form of natural language conditionals to their logical form. Finally, the thesis ends with a conclusion that briefly summarises the main conclusions drawn in its preceding chapters.