The concept of 'areal differentiation' is fundamental to geography and has indeed provided the theoretical justification for the study of 'regions' and the discipline of 'regional geography.' Areal differentiation, a term prominently associated with Richard Hartshorne, posits that every place or region on Earth is unique, possessing a distinctive character resulting from the particular combination and interrelationship of physical and human phenomena within its boundaries. It emphasizes that no two areas are exactly alike; each has its own unique 'areal character' or 'personality.'

This core idea underpins regional geography in several ways:

1. Focus on Uniqueness: Areal differentiation provides the rationale for regional geography's primary objective: to describe and understand the unique character of specific regions. Instead of focusing on universal laws or systematic patterns (as in systematic geography), regional geography delves into the idiographic—the particular and unique attributes of a place.
2. Integration and Synthesis: It mandates a holistic approach. To understand the unique character of a region, regional geography must integrate knowledge from various systematic sub-disciplines (e.g., climate, geomorphology, culture, economy, politics) and analyze how these diverse elements interact and combine to create the region's distinctiveness. This synthetic approach is central to regional studies.
3. Defining Regions: The concept helps in delineating regions. A region is not just an arbitrary spatial unit but an area that exhibits a certain degree of internal homogeneity or functional coherence based on the unique combination of its attributes, as identified through areal differentiation.
4. Historical Significance: Historically, regional geography, with areal differentiation as its philosophical cornerstone, was the dominant paradigm in the discipline. It provided a framework for comprehensive, place-based studies that sought to capture the essence of different parts of the world.

While systematic geography has gained prominence, the concept of areal differentiation remains crucial. It reminds geographers of the importance of context, the complexity of place, and the need for integrated understanding, thereby continuing to justify the study of regions as distinct entities worthy of detailed geographical inquiry for purposes ranging from academic understanding to practical planning and policy-making.