What is evidence

So much writing and speaking consist of piling one opinion on another with little or no evidence offered in support of any of them. there are some reasons for that. one is that the human mind is a veritable opinion factory, most of us have lots of opinions to share. another is that people tend to remember their opinions and forget the process by which they got them. Another, in some ways more significant, is that sometimes there is little or no evidence to remember. the opinion is based on nothing substantial. However, critical thinkers have developed the habit of checking the quality and quantity of the evidence before forming an opinion. they review their evidence before expressing an opinion.

kind of evidence

• personal experience:to evaluate it, ask, are the events typical or unique? are they sufficient in number and kind to support the conclusion?
• unpublished report:unpublished reports are stories we hear from others, often referred to as gossip or hearsay. it is difficult to confirm them. To evaluate it, ask, where did the story originate? How can I confirm that the version I heard is accurate?
• Published report:To evaluate it, ask, does the report cite the sources of all important items of information? Does the author have a reputation for careful reporting? which statements in the published report constitute evidence, and which should themselves be supported with evidence?
• Eyewitness testimony: to evaluate it, ask, what circumstances surrounding the event, including the eyewitness's state of mind, could have distorted his or her perception? what circumstances since the event could have affected the eyewitness's recollection?
• celebrity testimony:to evaluate it, ask, is the celebrity a paid spokesperson? does the celebrity offer any support for his or her views?
• expert opinion:to evaluate it, ask, does the person have, in addition to credentials in the broad field in question, specific expertise in the particular issue under discussion? one good indication for this is that the person does not just his or her opinion but also supports it with references to current research. Also ask whether the expert was paid.
• experiment:there are two broad types of experiments: the laboratory experiment and the field experiment. for a laboratory experiment, has it been replicated by other researchers? for a field experiment, have other researchers independently confirmed the findings?
• statistics:what is the source of the statistics? is the source reliable? how old the data? have any important factors changed since the data were collected?
• survey:was the sample truly representative? were the questions clear and unambiguous? were they objectively phrased rather than slanted? do other surveys corroborate the survey's findings?
• formal observation:there are two kinds of formal observational studies: detached observation(the observer not involved) and participant observation(the observer is involved). to evaluate it, ask, is is likely that the presence of the observer distorted the behavior being observed? was the observation of sufficient duration to permit the conclusions that were drawn? do the conclusions overgeneralize?
• research review:the reviewer examines all the scholarly studies that have been done and then summarizes and compares their findings. has the reviewer omitted any relevant research?

one additional question is applicable to all kinds of evidences: is this evidence relevant to the issue under consideration?

evaluating evidence

what constitutes sufficient evidence? here are general guidelines:

• evidence is sufficient when it permits a judgment to be made with certainty.
• if certainty is unattainable, evidence is sufficient if one view of the issue has been shown to have the force of probability.
• in all other cases, the evidence must be considered insufficient. in other words, if the evidence does not show one view to be more reasonable than competing views, the only prudent course of action is to withhold judgment until sufficient evidence is available.

what is argument

the word argument has several meanings, generally, there are three:

• a quarrel.
• the exchange of opinions between two or more people as occurs in a formal debate. in this sense of the term, an argument is ideally a cooperative endeavor in which people with different viewpoints work together to achieve a deeper, more accurate, understanding of an issue.
• the line of reasoning that supports a judgment. this definition is the one the book is focus on. we can think of an argument as a kind of verbal equation without mathematical symbols. just as the sum in a numerical equation may be composed of many numbers, so the conclusion of an argument may proceed from many premises(assertions). having an incorrect number in a column of figures will result in a wrong total, having an incorrect assertion will lead to a wrong conclusion. one important difference is that an argument is often more complex and difficult to test.

The parts of an argument: the premises and the conclusion. the basic principles used in evaluating arguments are as follows:

• the premises are either true or false;
• the reasoning that links the premises to the conclusion is either valid or invalid.
• correct premises plus valid reasoning equals a sound argument. either an incorrect premises or invalid reasoning will render an argument unsound.

evaluating arguments

the basic approach to evaluating arguments can be stated simply: decide whether the premises are true or false and whether the reasoning that leads from them to the conclusion is valid. if both criteria are met, the argument is sound.

to deal with some kinds of difficult situations, here are some guidelines:

• when an argument is longer than a paragraph, summarize it before asking and answering our questions.
• when we are uncertain which statements are the premises and which is the conclusion, ask ourselves exactly what idea the person is trying to get you to accept. then ask what reasons are offered in support of that idea.
• when an argument contains more than two premises, ask and answer our questions about each.
• when we are evaluating opposing arguments, neither of which is persuasive, look for a third alternative. often the alternative will be one that draws a little from each argument.
• when an argument contains hidden premises, identify them before proceeding with our evaluation.