Hello all. I have a question concerning the classification of simple machines, of which classically there are considered to be 6. I have joined the camp which claims there are only 2 true simple machine categories at the most fundamental level. The lever and the inclined plane. I will be happy to make the case in more detail as to why there are only these 2 if anyone is interested in more on that... (Some argue there are 4, but if you argue there are four I can't help but think you've made correlations in one place but decided to disregard similar correlations in other places) To the point now. Another form of mechanical advantage recently crossed into my awareness and had me questioning why it is not considered a distinct simple machine? The Hydraulic press. (pascals law). One answer I have found is that the 6 simple machines are 'classic' and that the press is just more modern. But in thinking more in terms of physics than history, would this not obviously be a simple machine? And if so, where would you place it (number 7 of 6 existing, number 3 of 2 existing, or within one of the existing 2/6? The similarity I see between wheel and axle, pulley, and lever is not apparent to me with the hydraulic principle. It seems distinct in large part due to its lack of fulcrum and rotational component. Similarly I can't draw any correlation between an inclined plane and the hydraulic piston. Lastly, I wonder if this can truly be discussed in terms of physics, or is it merely a question of engineering application, and where we draw the lines as to what is 'fundamental' is arbitrary?