While I believe that there is some truth to what both Locke and Mill had to say about words, I think that Locke's view better represents how words are used. I agree that words are used to represent the thoughts that we have in our mind. Before language occurs, it is generally necessary to have some idea in our head about the words we are going to use. Therefore, when we see something in the physical world, that physical object becomes and image in our mind. When we want to describe that object, we refer back to our mental image in order to produce the words necessary for an apt description.
I disagree with Mill for this reason, in that while we may describe something that occurs in the physical world, such as the sun causing day, I think what must first happen is for the knowledge of this physical event or object to enter our mind as an idea and then be spewed out as words. In particular, I think Mill's argument against words coming from ideas, in which he states that it is not the case that the idea of the sun causes the idea of day (in reference to the statement "the sun is the cause of day"), is not well-formed because he is breaking apart the sentence into separate ideas and separate constituents. The statement, "the sun is the cause of day," should be treated as one single idea and constituent which is thought of in the mind and then surfaced in the language of words.
Locke's view also has a few problems, in my opinion. First, I think Locke's argument that words can only stand for, "the ideas in the mind of him that uses them," is too broad of a generalization to be able to fully capture the use of words in our world. For example, if Person 1 says, "I like April," perhaps the idea in their head is that Person 1 likes someone named April. Person 2 listening, however, might have the idea that Person 1 likes the month of April. In this case, the word April either stands for a person or for the month. It can be argued that since Person 1 is the one that actually used the word, then Person 1's idea is the one attributed to the specific word April, meaning the name of someone in this case. However, we should also break apart what Locke means when he talks about the user of a word. Does this mean the person that originally used the word or could it mean all people that encounter the word? If a user includes anyone else that encounters the word, such as Person 2 who thinks April means month, then perhaps April can optionally stand for either the name or the month because these two users of the words have different ideas about them. In essence, I think that Locke's argument makes the most sense in regards to how words are used and interpreted, but I also think that it is a stretch for him to say that words can stand for
nothing but the ideas of its user.