Several meanings have been ascribed to the Doctrine:

1. That only those processes that have been observed to be in operation today operated in the past. The problem with this assumption is that humans are only acquainted with a very small slice of time. Even if echoes of the past brought down to us through religion and myth before the dawn of written history are included, our knowledge of the past is restricted to perhaps several tens of thousands of years or so. For a planet which has been in existence for four and a half billion years, that is only negligible moment. Can we be sure that in that brief instant of earth history with which we have direct experience, every type of process that has ever affected the earth has been in operation?

2. That the ranges of intensity of past processes was limited to those experienced by people. Investigation suggests that this interpretation is surely wrong. There is strong evidence of meteor impacts and spasms of volcanic activity in the past that vastly exceeded those of recent times.

3. That there is no net change through time. The change in life forms through time displayed by fossils, together with strong evidence that the compositions of ocean water and the atmosphere have evolved through time negate this interpretation.

4. That the physical laws and constants of the universe are unchanging. . That is, the 'rules of the game' have remained the same. Whatever the history of the earth has been, it has been consistent with those laws and constants. Gravitational and electrostatic attraction operated in the same fashion and with the same intensity 4.6 billion years ago, when the earth formed, as they do today. The speed of light was the same. Heat flowed from hot to cold places. And so on. This interpretation of the Doctrine is the most fundamental and remains unchallenged by scientists. If it is false, then knowledge of the universe by rational, scientific means seems impossible.

In a practical sense, although various levels of objection may be made to meanings 1-3, they provide a useful guide to developing explanations. Before invoking never-observed processes, or known processes operating with an intensity radically outside the range of human experience, hypotheses that invoke known processes with experienced intensities should be proposed and tested. Only when such 'hypotheses of the familiar' have been tested and retested and demonstrated to be inapplicable, should more exotic hypotheses be entertained. Experience has shown that in the vast majority of cases, 'hypotheses of the familiar' provide satisfactory explanation. The exotic should be reserved as a last resort.

Gaps in the rock record are indicated by physical discontinuities, such as a change in rock mineralogy, texture, orientation of layers, or fossil content . Some discontinuities are obvious, other subtle but recognizable, others may so subtle as to be essentially invisible.

• A simple, straight boundary between successive rock layers (e.g., layers 62 and 64) usually indicates a period of non-deposition.
• If the boundary is irregular, or underlain by a preserved soil layer, or if there are fragments of an older layer included within a younger layer (e.g., layers 53 and 62), removal of part of the rock record by a period of erosion is indicated.
• If there is a change in the orientation of the layers, such as between layers 8-11 and layer 52, tilting of layers may be indicated followed by removal of part of the rock record by a period of erosion.
• If there is minimal or no compositional or textural difference between successive layers (e.g., layers 51 and 53), the gap may not be recognized.

The nature of the organism itself plays a large role in determining whether it will be preserved. With rare exceptions, organisms that lack hard parts (shells, bones) are not preserved. For those that do have hard parts, the greater the number of individuals alive at any time increases the chances that some will be preserved. For example, the chances are greater that mice will be fossilized than elephants.

For fossils to be useful to paleontologists (those scientists who study the history of life), the fossils have to be observable and discovered. With the exception of fossils revealed by quarrying, tunneling, mining or drilling activities, the fossils have to be exposed at the earth's surface by removal of overlying material through weathering and erosion. That is, they have to be present at outcrops (places where rocks are exposed, such as storm swept shore areas, recently glaciated landscapes, steep cliffs, etc.) Exposure, however, subjects the remains to destruction by the same forces that revealed them: the erosional activities of running water, wind, the flow of glacial ice, landslides, and the actions of living organisms. There is, therefore, only a narrow window of opportunity for the discovery and investigation of fossils, between the moment of their exposure until the time of their destruction.

Erosion is not the only process that threatens fossils. If rocks containing fossils are subject to significant metamorphism, with rare exceptions the fossils will be destroyed. The greater the age of a rock, the greater is the likelihood that it and any fossils it contains will be radically altered as they are drawn down deep beneath the surface of the earth by collisions of tectonic plates. Luckily, some rocks almost as old as the earth itself have escaped complete transformation and a few retain evidence of extremely ancient, early life.

Fossils, therefore, provide an incomplete and biased record of life on earth. Organisms that lack hard parts are rarely preserved. For those that have hard parts, fossilization is still an unusual event requiring special circumstances, especially for land-dwelling creatures or those that exist in small numbers. Once formed, the possibility of fossils being available for observation depends upon their escaping destruction by normal, widespread, ongoing geologic processes. In consequence, the more time that has passed since the organism was alive, the less likely is it to persist in the rock record. Finally, discovery depends upon the fossil being located in outcrops or other locations accessible to investigators.

Despite these difficulties, a remarkable record of life on Earth has been preserved and studied by investigators. But it is understandable that there may, on occasion, be reasonable disagreements over how to interpret the record.