This page is adapted and expanded from the original work by the late Dr. Jonathan Dolhenty whose websites and writings have been expiring and/or malfunctioning following his death - some of his work is only available through The Internet Archive's Wayback Machine now. Wikipedia links have been added as a starting point to various words, the first-time they are mentioned. The source links have been put at the beginning of each major section in this combined series of essays about Logic and Critical Thinking. Screenshots have been taken of the original tables where it's not been possible to duplicate the colouring here.
Truth is the object of thinking. Some truths are obvious; others are difficult to acquire. Some judgments we make are simple; some judgments are complicated. Some arguments, whether made by us or others, may be straightforward and easily understood; other arguments may be complex and consist of a series of smaller arguments, each needing to be critically examined and evaluated.
Almost every object of knowledge has a branch of knowledge which studies it. Planets, stars, and galaxies are studied by astronomy. Chemistry studies the structure, composition, and properties of material substances and the transformations they undergo. The origin, evolution, and development of human society is the object studied by sociology. Economics, biology, geography, and grammar all have objects of knowledge which they investigate, describe, and try to explain.
Critical thinking involves a knowledge of the science of logic, including the skills of logical analysis, correct reasoning, and understanding statistical methods. Critical thinking, however, involves more than just an understanding of logical procedures. A good critical thinker must also understand the sources of knowledge, the nature of knowledge, and the nature of truth. But first, what is the science of logic?
The object of knowledge involved in the science of logic is "thinking," but it is "thinking" approached in a special way. Generally speaking, logic is that branch of knowledge which reflects upon the nature of "thinking" itself. But this may confuse logic with other branches of knowledge which also have the nature of "thinking" as a part of their specific object of investigation. We need a more detailed and accurate definition to eliminate any confusion.
Logic doesn't just deal with "thinking" in general. Logic deals with "correct thinking." Training in logic should enable us to develop the skills necessary to think correctly, that is, logically. A very simple definition would be: Logic is the subject which teaches you the rules for correct and proper reasoning. For those of you who want a more complete and "sophisticated" definition of logic, you can define it this way: Logic is the science of those principles, laws, and methods, which the mind of man in its thinking must follow for the accurate and secure attainment of truth. Take your choice.
We need to be aware of a distinction between what some call "natural logic" or common sense and "scientific" logic. We all have an internal sense of what is logical and what is not, which we generally refer to as "common sense." This "natural" logic we have learned from the moment of birth, through our personal experiences in the world and through our acquisition of language. Scientific logic, on the other hand, is simply our natural logic trained and developed to expertness by means of well-established knowledge of the principles, laws, and methods which underlie the various operations of the mind in the pursuit of and attainment of truth.
We have referred to the "science" of logic but logic is really more than just a science. The science part is the knowledge of the principles, laws, and methods of logic itself. This is important, to be sure. But logic must be put into action or else the knowledge provided within the science of logic is of little use. We can, therefore, also speak of the "art" of logic, that is, the practical application of the science of logic to our everyday affairs. Logic is not intended merely to inform or instruct. It is also directive and aims at assisting us in the proper use of our power of reasoning. In this sense, we can speak of logic as both a science and an art, a practical art meant to be applied in our ordinary affairs.
We want to be sure that we don't confuse the science of logic with the science of psychology. Psychology also studies "thinking," but it is a separate, autonomous discipline of its own. And logic is not a branch of psychology, but a separate discipline of its own. How are logic and psychology different?
The most obvious difference is that psychology is a "descriptive" science while logic is a "prescriptive" science. The difference between a descriptive science and a prescriptive science can best be illustrated by an example.
Let's suppose we are scientists and have been asked to study the differences between the American form of government and the British form of government. We find that in the United States there are three separate branches in the central government: the executive branch which includes the president, the legislative branch which includes the Senate and the House of Representatives, and the judicial branch which includes the Supreme Court. We discover that the president is elected by vote of the people, as are the senators and representatives, and that the judges of the Supreme Court are appointed by the president with approval of the Senate. Furthermore, we find that the president is both the ceremonial leader and the chief executive of the nation.
Now we turn our attention to England. We see that the Queen of England is not elected and functions primarily as the ceremonial leader of the country. Instead of an elected Congress, England has a parliament system, consisting of a House of Commons, which is elected, and a House of Lords, which is not elected by the people. Furthermore, we find out that the prime minister, who is the real head of the government, is not elected by the people, but is elected by the leading political party in the House of Commons.
What we have done in the above example is simply "describe" and report on each form of government, noting any similarities and differences between them. We have been functioning as "descriptive" scientists, in this case, as political scientists since governments are an object of knowledge of a scientific discipline called political science.
Let's suppose now that we go on to argue that England should adopt the form of government we have in the United States. In this case, we are no longer describing or reporting on a state of affairs. We are now recommending or "prescribing" how England should conduct its affairs when it comes to government. We have ceased to be scientists at this point and have become political philosophers. We are no longer being "descriptive," we have become "prescriptive."
Psychology is also a descriptive science. It is not primarily interested in how we "ought" to think but in describing how we actually think. It is interested in questions such as: Do men think differently from women? Do members of a primitive society think differently from members of an advanced civilization? What is learning and how can it be measured? What goes into the processes of thinking and learning? These questions call for descriptive answers.
Logic, on the other hand, is a prescriptive science, usually considered a branch of philosophy. It is interested in formulating the general rules for correct reasoning, prescribing how we must proceed if we are to argue clearly, consistently and, yes, logically.
Critical thinkers must be intimately acquainted with the concepts and methods of logic in order to be successful in activities involving critical thinking skills.
Most of the terms that are commonly used in logic will be fully described and explained in other sections of these essays on logic and as they become important to the discussion.
It may help us, however, to have some preliminary definitions to guide us on our way and it is advisable to point out at this time some limitations on the use we will make of some common words in English which can cause confusion. We need to realize that some words have several meanings in ordinary discourse and we need to be specific about how certain ordinary words will be used.
A complete discussion of the nature of the idea is undertaken here. You will notice then that we use the word "idea" somewhat differently from its ordinary, and many times misleading, meaning.
The word "idea" will be used to mean the intellectual representation of a thing. We consider the word "thing" to be the same as "being," the most general word that can literally apply to any actual or possible existent. In our ordinary conversations, we tend to use the word "idea" in a very broad sense to denote several things which we link together. Here we will be using the word very specifically. An "idea" will represent a single "thing," "single being," or "single existent," actual or potential.
The word "term" will be used as a "name" for the "idea." As we will see later, neither ideas nor terms are "true" or "false." Ideas simply "are," and terms are used to express them. Terms are simply sensible conventional signs which we use to express an idea.
The term "judgment" will be used to mean an act of the mind pronouncing the agreement or disagreement of ideas among themselves. The terms "true" and "false" apply only to a judgment. It is possible for a judgment to be merely an opinion if its state of certainty is in question. We should really refer to it then as an opinion, and not as a judgment.
A "proposition" is a sentence which expresses a judgment, either in speaking or writing. A proposition may be true or false, a determination which is actually made by the judgment which it represents. Propositions differ from other types of sentences such as questions, commands, and exclamations. Only propositions can be asserted or denied. An "argument" consists of propositions.
An argument is not a mere collection of propositions. An argument has a structure. We use the terms "premise" and "conclusion" when we talk about the structure of an argument.
The conclusion of an argument is that proposition which is affirmed on the basis of the propositions in the argument. This is what we are trying to show is true. This is what we want someone to accept at the end of any argument we may present.
The premises of an argument are those propositions which are used to provide the support or reasons for accepting the conclusion. These are what we show to justify our conclusion. These are an essential part of any argument.
It should be noted that premise and conclusion are relative terms. One and the same proposition can be a premise in one argument and a conclusion in another argument. This is one reason why many arguments can become complex and sophisticated. One argument, using one or more of the same propositions, can lead to another related argument using the same propositions. But, never fear. We'll learn how to deal with these multiple arguments.
In our ordinary everyday conversations, we tend to get sloppy with words, using the same words but with different meanings scattered throughout our speaking. One of the things that critical thinkers must do is to take words seriously and define them accurately. The words "reasoning" and "inference" are so important to critical thinking we need to make sure we understand how they will used during this study.
Reasoning is, first of all, a process. When we are engaged in reasoning in its simplest form, we are comparing two doubtful ideas with a third idea which we already know. If both doubtful ideas agree with this third idea, they also agree among themselves. If one of our doubtful ideas agrees with the third idea, and the other doubtful idea does not, then they also disagree among themselves. In the first case of reasoning, we have an affirmative conclusion, that is, we have affirmed the conclusion. In the second case of reasoning, we have a negative conclusion, that is, we have denied the conclusion.
The process of reasoning means that from certain things we already know to be true, we can acquire another truth not already known but that follows necessarily from those truths already known. It may seem strange to see it described this way, especially since reasoning is something we are constantly doing all day long. It's just that we are not consciously aware of what we are actually doing.
Reasoning and inference are sometimes thought to be the same process. This is true if we are talking about what is called mediate inference. Reasoning and mediate inference, which include deduction and induction (to be described later), are the same thought process. But there is another kind of inference called immediate inference, which some think is a primitive type of reasoning, wherein we draw a conclusion about something immediately without going through the process of thinking it out. Self-evident truths are an example of immediate inference.