The notion of an assumption is a familiar one, which for current purposes we define as follows:

    An assumption is a proposition which somebody takes to be true

    without having provided or considered evidence in relation to it.

Acknowledged, Stated and Unstated (Hidden) Assumptions

Assumptions vary in the degree to which we are aware of them and recognize them for what they are.

    ‘I’m not going to have chemotherapy,’ said Holly Hopeful. ‘You’ve got to believe that you’re one of the lucky ones. Besides, my herbalist, Dr. Wu, is giving me natural medicines. Dr. Wu is an excellent doctor.’

Here Holly is fully aware that her belief that she will be one of the lucky ones is an assumption. This is an acknowledged assumption, one that is recognized for what it is.

Holly says, explicitly, that Dr. Wu is an excellent doctor. However she does not seem to realize that she has not considered, or provided, any evidence for this. The assumption is stated, though not acknowledged as such.

Holly has not said this, but she apparently believes, without any basis, that Dr. Wu’s medical expertise extends to cancer and its treatment. This assumption is not even explicitly stated, let alone acknowledged. Assumptions like this are unstated. Such assumptions are sometimes described as hidden, though this term is often misleading in suggesting that the assumption was actively or deliberately concealed.

Where Assumptions are Found

As illustrated above, assumptions occur frequently in reasoning, and play a variety of roles. It is important to understand where they occur, because assumptions, by their nature, are potential points of vulnerability. More often than not, when an argument has a crucial weakness, it is one of the assumptions which turns out to be the problem. Places where assumptions are often found include:

  - Basic premises. Recall that a basic premise is one where the arguments ‘bottom out,’ i.e., one that has no further reasons or objections bearing upon it. Basic premises (at least, those with no basis) are classic cases of assumptions.

  - Unstated co-premises. A co-premise which has not been stated, has not, almost by definition, been provided with any evidence; as far as the audience is concerned, it is an unstated assumption.

  - Bases. Any given basis is generally surrounded by a cluster of assumptions, usually unstated. For example, Holly Hopeful’s basis for taking natural medicines is Dr. Wu’s (supposed) expert opinion that such medicines would help her heal. In doing so she is assuming that Dr Wu’s expertise is relevant to cancer and its treatment.

Identifying Assumptions

Identifying the assumptions in a piece of reasoning is one of the most important and yet difficult tasks in reasoning. It requires practice, experience, and lots of imagination to uncover all the significant assumptions, while also avoiding the mistake of attributing assumptions which are not in fact required.

In case of unstated co-premises, knowledge of the basic structure of reasons and objections can greatly assist in pinning down, in succinct form, exactly what assumptions are being made.

See also: Topics Proposition and Basis