Enter, penetrate, pierce, probe are comparable when meaning to make way into something so as to reach or pass through the interior.
Enter (see also ENTER 2 ) is the most comprehensive of these words and the least explicit in its implications. When the word takes a person for its subject, it often means little more than to go in or to go into, but sometimes it also suggests the beginning of a course of study, a career, or a proceeding. When enter takes a thing for its subject, it implies a making way through some medium and especially a dense or resisting medium.
Penetrate (see also PERMEATE ) carries a far stronger implication than enter of an impelling force or of a compelling power that makes for entrance and it also more often suggests resistance in the medium. It may imply either a reaching the center or a passing through and an issuing on the further side.
Penetrate, especially as an intransitive verb, often specifically takes as its subject something that is intangible or at least not objective but that has (in affirmative expressions) the power of making its way through.
Often also, as distinguished from the other terms, penetrate suggests the use of a keen mind or the exercise of powers of intuition or discernment in the understanding of the abstruse or mysterious.
Pierce in the earliest of its English senses implies a running through with a sharp-pointed instrument (as a sword, a spear, or a knife). In all of its extended senses it carries a far stronger implication than penetrate of something that stabs or runs through or of something that cuts into the very center or through to the further side.
Often the term imputes great poignancy or aesthetic effectiveness beyond what is usual to the thing that pierces.
Probe derives its implications from the earliest of its senses, to explore (as a wound, a cavity, or the earth) with a long slender instrument especially in order to determine depth, condition, or contents. In its extended senses it implies penetration so far as circumstances allow or so far as one’s powers or skills permit, and it usually suggests an exploratory or investigatory aim.
In some cases probe means little more than to investigate thoroughly (as by questioning those in a position to know facts).