Dopey Logic

'Can you make a sound and valid argument for the legalisation of Marijuana?'

The 'legalise it' debate is one that is regularly in the news these days. What are your views? What arguments do you back your views up with? In this activity, you can debate this with your class, survey yourself and your class (can be done online with google docs) and analyse the logical soundness of your arguments! The activity is an attempt to see the application of symbolic arguments in context. It can be challenging because context always invites us to lean one way or another but we must remember that, in logic, the propositions can be TRUE or FALSE and we can work with them either way.


The main aim of this activity is to provide an example of how symbolic logic can be used to model arguments in a real context. In doing so the aim is that students are able to examine their own arguments for logical flaws.


You will need to use the  Dopey Logic Questionnaire to record answers (this is also embedded below). The  results can be seen coming in! (take care to note when your results start as this may be used by a number of different people). Then you will need the  Dopey Logic activity sheet and the links to the following two articles about the legalisation of marijuana.  Why Marijuana should be legal and  Should Marijuana be Legalized under any Circumstances? Teachers can read more here,  Dopey Logic teacher's notes

Here there will be any multimedia and/or weblinks. Here there is a brief description of any further resource implications.

Syllabus links

This activity covers many of the main ideas in the syllabus concerning symbolic logic and, as such, is relevant to sections new, 3.1 and 3.2, previous, 3.4, 3.5, 3.6 and 3.7 of the syllabus. Conceptually it is probably more difficult than anything students will actually be asked to do in an exam, but the context and the exercise provides an excellent opportunity to practise working with symbolic logic.


Here follows an outline of what the task involves. If students are not reading this page then the teacher will need to show and give this overview.

  • Students and teachers should read the two sources given on the legalisation of marijuana.
  • Have a class debate on the issue, taking care to look for consistencies within arguments.
  • Fill in the questionnaire (this can be done online through this page or on paper)
  • Follow the activities on the worksheet that ask students to make truth tables for different compound logic statements and then check their own truth values against those from the truth table. This has the effect of testing the validity of their own arguments.
  • Students should reflect on the soundness of their own arguments before voting on the issue!

I did it my way!

As a practising maths teacher I know that most of us like to give activities our own little twist and do them 'our way'. It would be great to add a little collect of 'twists' from users. You can either add your twist to the comments section below or e-mail them directly to me at In time some of these twists may appear here....

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