Two ideas are fundamental to any system of classification: grouping and ordering. Grouping is the primary act of classification and one that is inherent in human thinking, as we saw in the introduction. We can all classify in this sense quite instinctively.

Grouping

This may seem to be a trivial exercise, but it serves to illustrate the essence of classification – the act of putting like with like and separating unlike – and it shows that at the broad level it is quite easy to do. Hopefully you were able not only to group the concepts, but also to identify the principles by which you did this, so that there was a logical and philosophical basis to your classification. The principle used to create a group is sometimes called the principle of division or characteristic of division: this is the technical term for the property or attribute that all the members of a group have in common.

Ordering

This process of grouping together related terms or concepts is central to a classification scheme, and it forms the first stage in constructing a classification. The second stage is to decide on the relationships between groups, since this determines the order in which the groups will be arranged.

Let's start by making some groups within the general area of vegetables. By putting like with like, we could organize the carrots, parsnips and turnips to make a class of root vegetables. Now we need to consider what will be placed next. We might want to position other groups of vegetables (leafy vegetables, and pulses) near to the roots, and then locate the whole class of vegetables near to other crops, such as fruits and cereals.