9. Teaching Deductive Reasoning in the Classroom

8 min readJun 22, 2019

My most recent blog outlined how we use as if sentences to teach students how scientific knowledge progresses via the logical structure of explanatory inference. In layman’s terms, this is called a guess.

This blog aims to offer similar ideas around how to teach students to reason deductively. The thought structures we use to help students deduce consequences from initial premises are if … then … sentences, which I discussed in an earlier post.

I hope that the following ideas will be useful both to those seeking to overhaul their KS3 curriculum as well as those thinking about their next GCSE intervention programme. The overall message of the series is that doing the former should ultimately negate the need for the latter at all.

The importance of being explicit

The first point to recognise is that deductive reasoning does not just happen naturally. To most science teachers, thinking in terms of cause and effect — how one thing affects another — is second nature. Many of us forget that this is something we once had to learn.

This is not helped by the fact that some students will arrive to school in Year 7 already confident in reasoning in this way. Whether they have picked up the habit of reasoning logically at home, in school, or from their own reading and research, some students just seem to get it.

Problems arise if we assume that there are those who just get it, and those who just don’t. The students who arrive at school seemingly unable to reach logical conclusions must be taught how to do so, otherwise we risk casting them aside and leaving them unable to think in one of the most powerful ways known to humankind.

Another way of putting this is as follows: if … then … reasoning is so fundamental to the scientific enterprise that it deserves to be taught explicitly. How should we go about this?

Concrete vs abstract premises

My first attempt to teach deductive reasoning explicitly was to use explain questions from GCSE papers. As described in the earlier post, scientific explanation relies on the notion of causality that if … then … sentences express succinctly. Why shouldn’t we just bring down the GCSE skill to our Year 7 and 8 lessons?

In some cases, this can work fine. In others, it can leave students feeling isolated and confused. What matters is how concrete the phenomenon being explained is.

Take examples around the body’s response to exercise. A GCSE question might ask why an athlete’s breathing rate increases during exercise. This is an experience that every student will have almost every day (certainly at Bobby Moore, where the science department is on the fifth floor).

Climbing five stories: a surefire way to increase breathing rate

Once they have been equipped with some conceptual apparatus around respiration and the lungs, most students are able to take the logical steps that lead them from the overall cause to the overall effect (more on this later).

Another example of a GCSE explain question might be: Explain how the structure of graphite allows it to conduct electricity. This question is much less suitable for KS3 students for two reasons:

There is a much wider gap between the phenomenon being observed and the student’s prior experiences (all students have been out of breath after exercise; fewer will be familiar with materials like graphite and the concept of electrical conductivity).
The complexity of the conceptual toolkit required to reach a conclusion is far greater than that in the breathing example (bonding is harder to get your head around than respiration).

In teaching deductive reasoning, then, it is essential that the students begin with concrete phenomena and move towards the abstract later, once the logic of if … then … is clear in their mind.

Likewise, if … then … sentences should be introduced with relatively simple concepts and expanded to more difficult ones later on. Only when deduction has become second nature can students comfortably begin with hypothetical premises in order to reach hypothetical conclusions.

Big questions

For this reason, our Year 7 curriculum is largely focused on the theory of particles in physics and the human body in biology. There are a few odds and ends of chemistry that are essential for later on (the idea of a chemical reaction; a simple treatment of the pH scale; common elements, compounds and mixtures), but generally we have sought to restrict what we teach early on to phenomena that lie close to hand.

We keep the phenomena concrete by framing these units around so-called ‘Big Questions’. Rather than the GCSE style question:

Explain why the volume of a gas increases when the number of air particles in the sample increases,

we ask:

Why does a balloon get bigger when you blow it up?

Instead of:

Explain how evaporation in a liquid causes the temperature of the remaining liquid to decrease,

we ask:

Why do hand sanitisers make your hands feel cold?

Each unit contains a bank of these big questions, which the students are asked to explain in a booklet. Time is given to each class to work through these booklets and explain all sorts of phenomena from the world around them.

Deploying scientific concepts to explain everyday phenomena

Avoiding the rupture

The aim of using concrete big questions is to avoid the dislocating rupture that Michael Young describes, whereby students are told to view the world in an entirely different way to their intuitive understanding.

Take the example of coldness. Our intuition tells us that leaving the window or fridge door open somehow transfers coldness to our skin. Students must look at lots of concrete examples involving hot things heating the surroundings before becoming comfortable with the idea that we feel cold because we heat the surrounding air or air in the fridge.

(Examples might include: Why does a hot cup of tea cool down? Why does turning the radiator on cause the surrounding air to get warmer? Why do hand sanitisers make your hands feel cold?)

To return to the landscape metaphor of an earlier post, if we go in with too much abstract knowledge too soon, it is akin to dropping a blindfolded child into the wilderness. Not recognising any of the landmarks, they are likely to feel lost and distressed.

We must introduce the features to them slowly and deliberately, always moving from something they already know to something just a little beyond. Most teachers claim to know this: after all, it’s been written by everyone from Vygotsky to Rosenshine.

It’s surprising nonetheless how many continue to toss students into the abstract wastes, then wonder why they’re unable to find their way home.

Mapping the curriculum

We have seen how early units must be framed around concrete phenomena. These phenomena must also be explicable using relatively simple concepts. The main concepts students have to appreciate to understand our first two Year 7 units are:

Particles — all matter is made up of tiny particles. The energies of a substance’s particles affect the properties and behaviour of the overall sample.
Human body — there are many structures in the human body which carry out a particular function. These structures have recognisable features which help them to carry out said function.

Neither of these is beyond the grasp of a Year 7 student. This is crucial; even if they are unfamiliar with the process of deductive reasoning, because the ideas are understandable they tend to feel comfortable using them as premises.

Our overall curriculum map therefore ensures that we do not include concepts that are too complex early on. In Year 8 we move on to Earth Science, Plants & Ecosystems and Forces & Space, each depending on a little more abstract thought than the one before.

In Year 9 we look at Chemical Reactions, Electromagnetism and Genes, all of which take the students well and truly beyond the everyday. By this point however, they should be secure enough in their reasoning ability to be comfortable juggling abstract ideas amongst their if … then … sentences.

Deduction in the classroom

In teaching deductive reasoning, I ask students to follow a process, outlined below. The example used is from the human body, which has the attraction of being relatively concrete to begin with.

If asked to explain, turn the sentence into a how / why question. e.g. Explain why the breathing rate increases during exercise.
Find the overall cause. What happens first? Do we breathe harder, then start exercising, or do we start exercising, then start breathing harder?
Find the overall effect. What happens last? See above.
Start the paragraph with If, followed by the overall cause: If we do exercise …
End the paragraph with then, followed by the overall effect: … then our breathing rate increases.
Work out what extra information is needed to fill in the gaps: exercise, energy, aerobic respiration, oxygen, breathing.
Fill in the gaps using if and then. Make sure to start each sentence with the end of the last sentence: If we do exercise, then our muscles need more energy. If our muscles need more energy, then …

(For those teachers who like acronyms, I christened this one QuIET: Question, If, Extra information, Then.)

Ideally, the students should be able to turn out something like the example below (on thermal conduction: why do metal saucepans often have plastic handles?). Some students will be able to write paragraphs like this after you’ve modelled the process once. For others, it will take years.

Like any model, this one has its limits. It cannot be used for every scientific question, and using if … then … sentences does not by itself imply understanding (see the example below). I hope it nonetheless offers a useful means of teaching the art of deduction, one of humankind’s most powerful logical tools.