The verb 'to conclude', along with the related noun conclusion, has two separate meanings. These are clearly linked, but some students fail to write good essays because they only think of the first meaning below. In Higher Education, the second is usually the more important meaning.
From an original Latin word concludere (~ 'to shut up' or 'enclose'), itself derived from claudere, which gives us the English verb 'to close', 'conclude' was basically a word of physical meanings - 'shut up', 'confine' or 'restrict' etc. This came to similar meanings in a less physical sense: OED gives meaning b as "to include, comprehend, comprise, sum up." This is OED's number I group of meanings, most of which are obsolete.
By the fifteenth century the meaning
- "To close any transaction; to end" (OED meaning II) was established. This is the simplest meaning of the conclusion to an essay or student assignment: that the piece of writing comes to an end.
This is not the best way to write an essay in Higher Education: that is provided by OEDs III group of meanings for conclude:
- "to come to a conclusion, infer, prove". It is this sense that is used in such phrases as "I conclude, therefore, my dear Watson...", a cliché often spoken in tribute to the detective Sherlock Holmes who solved his cases by the rigorous deduction of causes from the facts he observed. (A search using an on-line concordance does not find any occasion on which Conan Doyle, the author of the Sherlock Holmes stories, ever used this precise phrase.) More seriously, perhaps, Charles Darwin concluded from his observations of different species that evolution explained the variety to be found in nature.
- There is also a meaning in Law, Politics and History: 'to conclude a treaty, contract or similar agreement' is to settle it: to finish the negotiations and to have a single document which all parties are willing to sign.
The phrase in conclusion is better used with the second meaning above. It should be closer in meaning to 'therefore' than to 'lastly'; but it has elements of both.