"And because, the knowledge of the manner and Time to strike and defend, does of itself teach us the skill how to reason and dispute thereof only, and the end and scope of this Art consists not in reasoning, but in doing: Therefore to him that is desirous to prove so cunning in this Art, as is needful, It is requisite not only that he be able to judge, but also that he be strong and active to put in execution all that which his judgment comprehends and sees. And this may not be done without strength and activity of body: The which if happily it be feeble, slow, or not of power to sustain the weight of blows, Or if it take not advantage to strike when time requires, it utterly remains overtaken with disgrace and danger: the which faults (as appears) proceed not from the Art, but from the Instrument badly handled in the action.
Therefore let every man that is desirous to practice this Art, endeavor himself to get strength and agility of body, assuring himself, that judgment without this activity and force, avails little or nothing: Yea happily gives occasion of hurt and spoil."
And, because we're a culture of tidbits, here are a few culled from that passage above:
1) "The end and scope of this Art consists not in reasoning, but in doing."
2) "It is requisite not only that he be able to judge, but also that he be strong and active to put in execution all that which his judgment comprehends and sees."
3) "Let every man that is desirous to practice this Art, endeavor himself to get strength and agility of body."
Essentially, di Grassi is telling us outright that if we can't do something that the Art is telling us we should be able to do, we need to condition our body to be stronger, faster, and more agile so that we can execute what the Art requires. The inability to execute a technique doesn't lay fault at the Art, but rather at the person's body who's trying to execute the technique.
This is a curious distinction, because we quickly recognize that it's important to consider the use of pronouns at the end of the first quoted paragraph above. The "strength and activity of body" is the subject of the sentence, and the succeeding repetitive use of the pronoun "it" then refers to the body specifically. This is an interesting construction, as one's body is the focus, not one's personhood. With this important distinction in mind, we see that according to di Grassi, the body is at fault for very specific shortcomings when we train in this Art, which are:
1) the body's inability to "sustain the weight of blows" due to being "feeble, slow, or not of power"
2) the body's inability to "take not advantage to strike when time requires"
These two shortcomings can be broken down into three physical attributes that must be developed if we're to be effective fencers who wish to accurately reconstruct this Art. The first shortcoming listed above deals with all three attributes: endurance, strength, and agility. The phrase "to sustain the weight of blows" is referring to the continual motions of the fight, the engaging and reengaging that continual happens during a violent encounter. Note that di Grassi is very specific here--he uses the plural, "blows," because a fight is rarely finished after a single strike. In order to then meet and deliver repeated blows, one must condition their body to be able to do so. Conditioning obviously builds our endurance.
The second attribute that can be ascertained from that first shortcoming of the body is strength. di Grassi tells us outright that if our bodies are "feeble" and "not of power" that we will be unsuccessful in utilizing the Art. Strength, then, is an important component to our training. Of course, tied together in the same sentence with the attribute of strength is the third attribute of agility, which di Grassi's translator simply identifies with being "slow."
What's interesting is the component of timing mentioned in the second shortcoming of the body, and this relates directly to agility. This is interesting because di Grassi wasn't satisfied with saying that our body can be slow, but expounded on the crucial element of a fight that a slow body impacts. Timing is not just a function of recognizing that an opening exists or has been created, but coupling that recognition with a body that's physically capable of acting within that small window of opportunity.
di Grassi also doesn't equivocate on the ramifications of not training our bodies. He states that without proper physical training, the body is "overtaken with disgrace and danger." According to this 16th century master, not training one's body is a literal disgrace to the Art, not to mention dangerous.
Finally, di Grassi ends that first paragraph quoted above with a pregnant double meaning. He states that the faults of inability lie not with the Art, but with the "Instrument badly handled in the action." As a fencing treatise, "instrument" could obviously mean the weapon at hand, or being "handled," but given the context of the subject before this final admonition, "instrument" can also be read as "body." Therefore, the admonition reads this way: your body is your instrument--do not handle it badly, but rather hone and train that instrument so that you wield it with endurance, strength, and agility. Without a focus on those physical elements, di Grassi states that we will often experience "hurt and spoil," or in modern parlance, pain and ineffective fighting.
So, train your body hard! In fact, stop reading this and do some interval training: burpees (endurance), followed by a winding drill in response to pressure (agility), followed by pushups (strength).
We're listening di Grassi, we're listening.
