The comparison between the Crusades and the September 11 terrorists in a Victorian textbook was deliberately provocative and designed to spark debate, teachers said yesterday.

The idea that the Crusaders and their fight in a holy war shared similarities and "moral equivalence" with the September 11 terrorists was intended to teach students how to support an argument, educators said.

The book, Humanities Alive 2 developed for Year 8 students, was criticised this week by Melbourne University historian Barry Collett for being historically inaccurate and misleading in its depiction of the Crusades and the church during the Middle Ages.

Victorian president of the Australian Education Union Mary Bluett said the text relating the Crusades and September 11 was purposely provocative to spark discussion and tease out ideas from students.

"Clearly there's sensitivity around it and teachers as professionals would handle that sort of debate very carefully," she said.

Ms Bluett said the aim of the exercise was not to teach students that there were similarities between the Crusades and September 11, but to teach them the principles of mounting an argument.

"It's really about teaching young people to analyse the words being said, think about their response and justify their response. It's a tool for teaching them how to advance an opinion and back it up," she said.

Director of the teaching resources and textbook research unit at Sydney University Michael Horsley said how the textbook was used by teachers was more important than its content. "It isn't a matter of what's written on paper. Any text can be interpreted in many different ways by children - and that's where the teacher's knowledge and expertise comes in," he said.

He said history in Victoria was taught as part of a combined society and environment syllabus with geography and economics, rather than a stand-alone subject. As a result, textbooks covering all three subjects were necessarily simplified or carried little detail.

"This is not a history course; kids aren't necessarily studying medieval history for a longer period of time," he said.

Mr Horsley added that most commercial textbooks were written in conjunction with teachers to reflect the way they taught the syllabus, and were subjected to peer review.

President of the History Teachers Association of Australia Nick Ewbank said the accuracy of textbooks was safeguarded by market forces.

"Textbooks which are generally regarded as poor quality won't sell and the invisible hand of the market will operate," he said. "I don't see there's any huge virtue of having a group of bureaucrats or teachers sitting down to review every textbook that comes out."