The transition to the dietary laws of marine life introduces a distinct shift in classification. Unlike birds, which are typically identified by specific species, or land animals, fish lack individual names in the Holy Tongue [חומת אנך]. Because they cannot be listed by name, clear physiological markers are provided to distinguish the permitted from the forbidden [רלב״ג].
The required identifiers are fins and scales. Fins are the organs that enable a fish to swim [רש״י, רמב״ן, ביאור יש״ר], functioning as an appendage or branch connected to the body [רש״ר הירש]. Scales serve as the permanent covering over the fish's skin [רש״י]. The primary approach among commentators is that not every hard shell or armor qualifies as a scale. To render a fish pure, its scales must be removable from the skin, either by hand or with a tool, much like clipping nails or the overlapping plates of military armor [רמב״ן, רש״ר הירש, רד״צ הופמן, תורה תמימה]. This specific requirement excludes amphibious creatures with rigid, unpeelable shells [רד״צ הופמן], but permits fish whose scales naturally shed when they are pulled from the water [שטיינזלץ].
The Sages established a biological principle that any aquatic creature possessing scales also possesses fins, meaning the presence of scales alone is technically sufficient to confirm a fish's purity. This raises the question of why fins are explicitly demanded at all. Some explain that detailing both signs serves to magnify the Torah, formulating the law with absolute clarity to eliminate any potential doubt [תורה תמימה]. Alternatively, others suggest that while the Torah generally addresses the absolute majority of cases, rare species of fish might exist in nature that possess scales but lack fins. To prevent the consumption of these anomalies, both signs are explicitly required [הכתב והקבלה].
The underlying reason for these specific signs is intimately connected to the creature's habitat and its subsequent effect on human health. Fish with fins and scales typically inhabit the upper, clear layers of water. Exposed to air and sunlight, they develop a natural body heat that repels excess moisture. In contrast, creatures lacking these features tend to dwell in murky, muddy riverbeds. Their bodies remain cold and damp, and consuming them introduces harmful, cold moisture into the human body [רמב״ן, רבנו בחיי, צאינה וראינה].
The specified habitats include seas, referring to the large, salty bodies of water formed during creation, and rivers, denoting fresh, flowing water [רלב״ג, ביאור יש״ר]. The specific emphasis on water establishes that the requirement for fins and scales applies exclusively to natural bodies of water that rest on the ground and naturally spring or flow. Conversely, water gathered in artificial vessels, closed pits, or caves is not fully subject to this prohibition. Consequently, one may drink from such vessels without fearing the accidental ingestion of tiny, finless, and scaleless creatures that developed inside, provided those creatures never left the water [תורה תמימה, מלבי״ם, רש״ר הירש, רד״צ הופמן]. Furthermore, a permitted creature must be caught while still in its aquatic environment; if it crawls onto dry land of its own accord, it is classified as a creeping creature of the earth and becomes forbidden [העמק דבר].
The explicit permission to consume these specific creatures also introduces laws regarding mixtures. It allows the consumption of a pure fish found whole inside the stomach of an impure fish that swallowed it, or a pure fish pickled alongside impure ones, provided it has not dissolved or absorbed their essence. This highlights that only the pure fish itself is permitted, strictly distancing it from any impure elements [תורה תמימה, מלבי״ם, צפנת פענח].
On a symbolic level, water represents the Torah. A scholar navigating the vast sea of Talmud must possess both fins and scales. This requires not only the intellectual agility to navigate complex texts but also the protective armor of good deeds and the fear of heaven. Studying under someone who lacks these essential traits results in acquiring a superficial Torah that fails to inspire genuine, righteous action [שפתי כהן].