According to Frege, "By a thought I understand not the subjective performance of thinking but its objective content which is capable of being the common property of several thinkers" (Frege, 1952,62). Further he states, "Every sentence has a thought because the thought is in itself immaterial clothes found in the material garment of a sentence and thereby becomes comprehensible to us" (Frege, 1956, 292). Thus every sentence expresses a thought. Since every statement expresses thought, they state about the facts of the world. So there is a correspondence between the sense of a proposition and the facts of the world. Hence, all propositions can be judged as either true or false. To put it in Frege's words, "by truth-value of a sentence I mean the circumstances that it is true or false" (Frege, 1952, 63). Thus, to understand a proposition means to know what is the case if it is true.

Frege holds that different propositions may express the same thought. In other words, a given thought can be adequately expressed by a number of structurally dissimilar sentences, yet their truth-value will remain same. For example,

1. The drawer is empty.
2. There is nothing in the drawer.
3. The number of things in the drawer is zero.

These three sentences differ in their structure. Some are quantified and others are not, some are expressed in negative terms and others are not, some are having n-ary predicate and some are not, yet all those sentences express one and the same thought. It is so because anyone who has grasped the thought of the above three propositions must be able to recognize one as true only if (s)he recognizes the others as true. This is possible because the content of the propositions are the same. A few examples of this similar type are:

1. 1. For every X, T(x) ⇔ M(x).
   2. The value range of T = the value range of M.
2. 1. Line X is parallel to line Y.
   2. The direction of line X is the direction of line Y