## Logic and terminology

**What is a necessary condition?**

**How is a necessary condition different from a sufficient condition?**

**How can the necessary condition logically re-formulated as a sufficient condition?**

**What is a dichotomous necessary condition?**

**What is a discrete necessary condition?**

**What is a continuous necessary condition?**

**What is a necessary condition “in kind”?**

**What is a necessary condition “in degree”?**

**What is a necessary configuration?**

**What is a necessary AND configuration?**

**What is a necessary OR configuration?**

**What is a necessary condition?**

A necessary condition is a (level of a) condition that must be present to have a given (level of the) outcome. Without the condition the outcome will not occur.

**How is a necessary condition different from a sufficient condition?**

A necessary condition allows the outcome to exist; a sufficient condition produces the outcome.

**How can the necessary condition logically re-formulated as a sufficient condition?**

A necessary condition can be formulated in two ways:

- The presence of the condition is necessary for the presence of the outcome
- The absence of the condition is sufficient for the absence of the outcome

If we use “X is necessary for Y” then we mean that the presence of X is necessary for the presence of Y.

**What is a dichotomous necessary condition?**

A dichotomous necessary condition is a necessary condition that can have only two discrete levels (0/1, absent/present, Low/High, etc.). The outcome can have any number of levels (dichotomous, discrete or continuous).

**What is a discrete necessary condition?**

A discrete necessary condition is a necessary condition that can have more than two discrete levels, for example trichotomous (0/1/2, absent/neutral/present, Low/Medium/High, etc.). The outcome can have any number of levels (dichotomous, discrete or continuous).

**What is a continuous necessary condition?**

A continuous necessary condition is a necessary condition that can have an infinite number of levels between a minimum value and a maximum value. The outcome can have any number of levels (dichotomous, discrete or continuous).

**What is a necessary condition “in kind”?**

A necessary condition “in kind” is a qualitative expression of necessity of the condition for the outcome: “X is necessary for Y”.

**What is a necessary condition “in degree”?**

A necessary condition “in degree” is a quantitative expression of necessity of the condition for the outcome: “X=X_{c} is necessary for Y=Y_{c }”, where X_{c} and Y_{c} are specific levels of the condition and the outcome, respectively.

**What is a necessary configuration?**

A necessary configuration is a combination of conditions that is necessary for an outcome (necessary AND configuration, or necessary OR configuration).

**What is a necessary AND configuration?**

A necessary AND configuration is a necessary configuration where each single condition is necessary. For example, Apple and Flour are single necessary conditions and both are necessary for an apple pie. A combination of single necessary conditions is a necessary AND configuration. A necessary AND configuration consists of single nececessary conditions.

**What is a necessary OR configuration?**

A necessary OR configuration is a necessary configuration where the conditions are substitutable. For example the combination Green apple or Red apple is necessary for an apple pie, but the separate conditions are not necessary (the conditions are substitutable). A necessary OR configuration may indicate that a higher level condition is necessary (Apple).