Émile Durkheim, a foundational figure in sociology, introduced the concept of 'social facts' as the proper subject matter of sociology. He defined social facts as ways of acting, thinking, and feeling, external to the individual, and endowed with a power of coercion by virtue of which they control him. While emphasizing their objective reality, Durkheim also acknowledged several problems in observing these unique phenomena.

1. Invisibility and Immateriality: Unlike physical objects, social facts are not tangible or directly observable through sensory perception. They exist in the collective consciousness, as collective representations, norms, values, and institutions. This makes their direct observation challenging, as one cannot 'see' a moral rule or a collective sentiment in the same way one sees a tree.

2. Coercive Nature is Often Indirect: The coercive power of social facts is often subtle and indirect. It's felt through social pressure, sanctions, or the consequences of non-conformity, rather than overt force. This makes it difficult to pinpoint and observe the exact moment or mechanism of coercion.

3. Subjectivity of the Observer: Researchers are themselves members of society and are thus subject to the very social facts they are trying to observe. This internalization of social facts can make it difficult to maintain objectivity and treat them as 'things' external to oneself, as Durkheim advocated. The researcher's own biases and preconceived notions can interfere with objective observation.

4. Difficulty in Distinguishing from Individual Manifestations: Social facts are collective phenomena, but they manifest through individual actions and beliefs. It's challenging to differentiate between a truly collective social fact and its individual psychological expression. Durkheim warned against reducing social facts to individual psychology, emphasizing the need to look for collective indicators.

5. Measurement Challenges: Quantifying or measuring abstract concepts like collective sentiments, moral density, or institutional structures is inherently complex. Durkheim himself resorted to indirect indicators, such as suicide rates (to study anomie) or legal codes (to study types of solidarity), acknowledging the difficulty of direct measurement.

To overcome these problems, Durkheim proposed treating social facts as 'things' – objective realities that can be studied empirically, much like natural scientists study natural phenomena. He advocated for comparative methods, statistical analysis, and the study of external indicators (like laws, statistics, and social institutions) to infer the existence and influence of social facts, rather than relying on introspection or individual accounts.