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MUSE | Natural Selection | Unit Overview and Materials | Section 2: Comparing Explanatory Models | 2B: Understanding Paley's Model | Instructional Notes


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Learning Outcomes

Paley' s Model

  • Paley's model invokes an intelligent designer to account for the apparent perfection of organisms' adaptations to their environments
  • Paley believed species to be fixed and unchanging, each having an "essential" type that set it apart from all others


  • Models are ideas that scientists use to explain patterns they see in the world
  • Models can be compared based on data, the inferences made in the model, and the prior knowledge and beliefs upon which they are based

  • Excerpt from Natural Theology by William Paley
  • Reading Questions: Paley's Argument from Design
  • Cow Eyes for dissection (optional)
  • Eye dissection lab protocol

Questions: Paley's Prior Knowledge and Beliefs

Time Frame and Sequence

This activity will take 3 full class periods and a few minutes on an earlier day to introduce the task. Students will need to do some of the reading at home.

Prior to Day One

Pass out the reading to students with a few minutes remaining in class. Tell students that in the coming days they will be reading the original writing of three men who each, in their own ways, offered explanations for the amazing adaptations and species diversity that we see on earth. The goal of this set of activities is to examine the arguments offered by each writer by considering the phenomena they were trying to explain, the data they brought to bear in their explanations and the prior knowledge and beliefs under which they were operating. This framework builds directly on the one developed during the Cartoon Activity.

Explain to students that the readings themselves will be difficult to read, largely because they were written in the late 18th and early 19th centuries. It may be worthwhile to spend a few minutes with the Paley Reading at the end of the class period to introduce some strategies for reading complicated texts. You may ask them to think about how they read texts such as these in their literature classes. Encourage students to underline key ideas, write questions or comments in the margins, and use a dictionary to look up unfamiliar words. Assign the rest of the reading and the first set of Reading Questions for homework.

Day One

Begin the discussion of the Paley reading by giving students an opportunity to ask any general questions they may have. Frequently, students will be confused by some of the vocabulary or by the way that Paley makes his points. Provide a chance for students (with assistance from you) to struggle through some of the difficult areas in the reading. Encourage students to help each other rather than simply providing answers quickly. Part of the point of this activity is for students to use the framework developed during the cartoon activity to analyze arguments.

Once general questions have been answered, move on to a more pointed discussion of the Reading Questions. These questions ask students to identify the phenomenon Paley is trying to explain, the data he uses in his argument, and the explanation he develops. Students are also asked to explain the analogy that he uses in the paper. The discussion of these questions will probably take the remainder of the hour.

Day Two

In our class, this day is usually used for eye dissection, however, this can be omitted. Alternatively, students can be shown diagrams of the eye and engage in a discussion of the ways in which it is particularly well suited for its function.

Begin by reviewing with students the analogy between the eye and the telescope that Paley made in his writing. Ask students to point out in the reading a few of the structures that Paley described as being so well suited for their function. Explain to students that they will be getting an opportunity to examine many of those structures as they dissect cow eyes. Pass out the Eye Dissection Lab Protocol and go over safety precautions for the procedure. Give a brief overview of the procedure and then allow groups to work. Remind them that the point of this activity is for them to gain an appreciation for the complexity of the eye. Encourage students to use the protocol that is provided to avoid damaging structures like the iris and the lens. Point out the questions at the end of the lab which ask them to write about particular structures. This dissection with clean-up will probably take the whole class period.

Day Three

This class period can be used to wrap up the discussion of Paley's argument. Begin by asking students to share some of the observations they made about the cow eyes yesterday. Then explain that at this point they are going to try to imagine what Paley's prior knowledge and beliefs were. Pass out the questions on Paley's Prior Knowledge and Beliefs. These questions require students to "think like Paley" in order to try to come up with some of the beliefs about species and the potential for change that he might have held. Allow students to work on the questions in groups for about 15-20 minutes. Once they have had a chance to consider the questions call the group back together and go over the questions as a class. The main goal of these questions is to get students to realize that Paley did not believe that species could change over time and that he clearly held a strong belief in a creator. These beliefs played a large role in the argument he developed.

Teaching Strategies and Student Ideas

Paley Reading

The writing style of Paley is sometimes difficult for students to follow. We have found that spending a few minutes working with students to identify key ideas at the beginning of the reading is very helpful to them as they read the remainder of the excerpt. Many students are surprised that they are being asked to read an original piece of writing in a science class. Remind them that one goal of this activity is to come to understand the model that Paley proposed and that reading his own writing is an important part of the process. Encourage students to write on their copies of the reading, underlining or highlighting, or writing questions in the margins. These notes and markings will help them during the class discussions.

Analyzing the argument

Students will have some difficulty with the argument of Paley. The big ideas that you should try to highlight are that generally he was trying to explain why organisms seem to be so well adapted to their surroundings-this is the phenomena. He provides several examples of such adaptations and so the data that he based his argument on came from his observations of the natural world. It may help to talk about naturalists in the 18th and 19th centuries and the importance they placed on collecting specimens and observing nature. His explanation draws heavily on the analogy between the eye and the telescope. He develops the idea that the telescope is well-suited to its purpose as the result of good engineering by a "contriver" and connects that to the amazing engineering he sees in the eye. Thus, if the telescope is thought to have been made specifically for its purpose by an intelligent designer, so too must the superior design of the eye be considered the result of a similar process. With the use of this analogy, the basic explanation he develops is that there is a "contriver" or creator that has made organisms in particular ways to suit their specific purposes.

Cow eyes

Students are generally very fascinated by the cow (or sheep) eyes. We do this activity for two reasons. First, it allows students first hand experience with the eyes so they can see for themselves many of the structures Paley described. Second, we find that this is a nice break from the whole class discussions that have been taking place. Take some time at the beginning of the period to orient students to the structure of the eyes so that when they do the dissection they will be more likely to see these structures.

We recommend that you use fresh eyes so that the lens and humors will be clear. With preserved eyes these structures often become cloudy. During the actual dissection, students are sometimes hesitant to cut into the eyes with the required force and so they end up taking a long time to complete the first cut. The sclera is very tough, and students should be told that they will need to use a fair amount of force to make their way into the eye. We have found that dissecting scissors are better for this purpose than scalpels, but either one will work.

Paley's Prior Knowledge and Beliefs

Students may have more success thinking through Paley's prior knowledge and beliefs if they are allowed to discuss their ideas with their peers. This is a difficult task and one that requires students to do more than simply go back to the reading and pull out quotes. Instead, these questions ask students to use evidence from Paley's arguments to imagine what his belief structure may have been like and then use that set of beliefs to answer the questions as they think he might have answered them. For example, the first question asks students whether Paley viewed species as fixed or malleable. In other words, did he think things changed over time? If we are "thinking like Paley" we would say that he probably viewed them as fixed because of his strong belief in a perfect creator. There is not a place in the text that says this outright, however. It may be useful to go over the above example with the class as a whole so they do not become frustrated trying to "find" answers in the text.

The same set of questions is used for each of the readings (Paley, Lamarck, and Darwin). This will aid in comparison later on, but means that some of the questions may seem a little awkward at this point. For example, the question about the importance Paley placed on variations within a species is not directly addressed in the reading, but is an important distinction between Paley and Darwin. Encourage students to imagine what Paley might have thought about the issue. It seems unlikely that he thought these small differences were very important given that he did not discuss them and given his beliefs about the role of a "contriver".

We have used Paley's model in our class because it provides an opportunity to talk about a model that is based on the belief in a supernatural force. Paley's model is not intended to represent or encompass the ideas of modern creationist teachings, but instead it provides a nice opportunity to discuss the metaphysical commitments that modern science operates under. Discuss with students how in modern times scientists seek to explain the world with models based on naturalistic assumptions. These conversations will continue when students examine all three models during the Three Model Comparison (material 2E) portion of this section.


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